Wireless communication system, wireless communication apparatus and wireless communication method and computer program

ABSTRACT

In order to solve problems arising when a communication system such as a wireless LAN is constructed as a decentralized distributed type network without a relationship of control station and controlled stations such as a master station and slave stations, in a wireless communication system composed of a plurality of communication stations without a relationship of control station and controlled stations, respective communication stations transmit beacons with information concerning a network written thereon with each other to construct the network, and it becomes possible to make sophisticated judgment such as communication states of other communication stations by those beacons.

TECHNICAL FIELD

The present invention relates to a wireless communication system, awireless communication apparatus and a wireless communication method anda computer program suitable for use in configuring a wireless LAN (LocalArea Network: local area network) for making data communication, forexample, to construct a decentralized distributed type network without arelationship of control station and controlled station, such as a masterstation and slave stations.

More specifically, the present invention relates to a wirelesscommunication system, a wireless communication apparatus and a wirelesscommunication method and a computer program for forming a decentralizeddistributed type wireless network formed when respective communicationstations transmit their beacons with network information and the likewritten therein with each other at every predetermined frame period, andparticularly relates to a wireless communication system, a wirelesscommunication apparatus and a wireless communication method and acomputer program for forming a decentralized distributed type wirelessnetwork while avoiding collision of beacons transmitted from therespective communication stations.

BACKGROUND ART

As media access control for wireless LAN system, access controlstandardized by IEEE (The Institute of Electrical and ElectronicsEngineers) 802.11 systems have been widely known so far. InternationalStandard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std 802.11, 1999 Edition,Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer(PHY) specifications or the like has described the details of theIEEE802.11.

Networking in the IEEE802.11 is based on a concept of a BSS (BasicService Set). Two kinds of BSS are available, that is, BBS defined bythe infrastructure mode in which a master control station such as anaccess point (Access Point: AP) exists and IBSS (Independent BSS)defined by the ad hoc mode composed of only a plurality of mobileterminals (Mobile Terminal: MT).

Operations of the IEEE802.11 in the infrastructure mode will bedescribed with reference to FIG. 30. In the BSS in the infrastructuremode, an access point for performing coordination should be absolutelyprovided within a wireless communication system. In FIG. 30, assumingthat a communication station SAT0, for example, is a communicationstation SA which functions as an access point, then BSSes within a rangeof radio waves near the local station are collected to construct a cellin the so-called cellular system. Mobile stations (SAT1, SAT2) existingneat the access point are accommodated into the access point and joinedthe network as a member of the BSS. The access point transmits a controlsignal called a beacon at a proper time space. A mobile terminal thatcan receive this beacon recognizes that the access points exists near itand establishes connection between it and the access point.

The communication station SAT0, which is the access point, transmits abeacon (Beacon) at a predetermined period space as shown on theright-hand side of FIG. 30. The next beacon transmission time is sentinto the beacon by a parameter called a target beacon transmit time(TBTT: Target Beacon Transmit Time). When a time reaches the TBTT field,the access point activates a beacon transmission procedure. Also, sincea neighboring mobile terminal receives a beacon and is able to recognizethe next beacon transmission time by decoding the inside TBTT field,depending on the cases (mobile terminal need not receive information),the receiver may be de-energized until the next TBTT field or aplurality of future target beacon transmission times and the mobileterminal may be placed in the sleep mode.

This specification principally considers the gist of the presentinvention in which the network is operated without application of amaster control station such as the access point, and hence theinfrastructure mode will not be described any more.

Next, communication operations according to the IEEE802.11 in the ad hocmode will be described with reference to FIGS. 31 and 32.

On the other hand, in the IBSS in the ad hoc mode, after eachcommunication station (mobile terminal) has negotiated with a pluralityof communication stations, each communication station defines the IBSSindependently. When the IBSS is defined, the communication station groupdetermines the TBTT at every constant interval after negotiations. Wheneach communication station recognizes the TBTT with reference to a clockwithin the local station, if it recognizes that other communicationstation has not transmitted the beacon after a delay of a random time,then the communication station transmits the beacon. FIG. 31 shows anexample of the case in which two communication stations SAT1, SAT2constitute the IBSS. Accordingly, in this case, any one of communicationstations belonging to the IBSS is able to transmit the beacon at eacharrival of the TBTT field. Also, it is frequently observed that thebeacons will conflict with each other.

Further, also in the IBSS, according to the necessity, eachcommunication station is placed in the sleep mode in which a powerswitch of its transmission and reception unit is turned off. A signaltransmission and reception procedure in this case will be described withreference to FIG. 32.

In the IEEE82.11, when the sleep mode is applied to the IBSS, a certaintime period from the TBTT is defined as an ATIM (Announcement TrafficIndication Message) Window (hereinafter referred to as an “ATIMwindow”).

During the time period of the ATIM window, since all communicationstations belonging to the IBSS are operating the reception units, eventhe communication station which is being operated in the sleep modefundamentally is able to receive communication in this time period. Wheneach communication station has its own information for othercommunication station, after a beacon has been transmitted in the timeperiod of this ATIM window, the communication station lets the receptionside know that the communication station has its own information forother communication station by transmitting the ATIM packet to othercommunication station. The communication station, which has received theATIM packet, causes the reception unit to continue operating until thereception from the station that has transmitted the ATIM packet isended.

FIG. 32 shows the case in which three communication stations STA1, STA2,STA3 exist within the IBSS, by way of example. As shown in FIG. 32, atthe time TBTT, the respective communication stations STA1, STA2, STA3operate back-off timers while monitoring the media state over a randomtime. The example of FIG. 32 shows the case in which the communicationstation STA1 transmits the beacon after the timer of the communicationstation STA1 has ended counting in the earliest stage. Since thecommunication station STA1 transmits the beacon, other two communicationstations STA2 and STA3 do not transmit the beacon.

The example of FIG. 32 shows the case in which the communication stationSTA1 holds information for the communication station STA2, thecommunication station STA2 holding information for the communicationstation STA3. At that time, as shown in FIGS. 32B, 32C, after havingtransmitted/received the beacons, the communication stations STA1 andSTA2 energize the back-off timers while monitoring the states of themedia again over the random time, respectively. In the example of FIG.32, since the timer of the communication station STA2 has ended countingearlier, first, the communication station STA2 transmits the ATIMmessage to the communication station STA3. As shown in FIG. 32A, whenreceiving the ATIM message, the communication station STA3 feeds themessage of the reception back to the communication station STA2 bytransmitting an ACK (Acknowledge) packet which is an acknowledge packetto the above communication station. After the communication station STA3has finished transmitting the ACK packet, the communication station STA1further energizes the back-off timer while monitoring the respectivestates of the media over the random time. When the timer finishescounting after a time set by the timer has passed, the communicationstation STA1 transmits the ATIM packet to the communication stationSTA2. The communication station STA2 feeds the message of the receptionback to the communication station STA1 by returning the ACK packet tothe above communication station.

When the ATIM packet and the ACK packet are exchanged within the ATIMwindow, also during the following interval, the communication stationSTA3 energizes the receiver to receive information from thecommunication station STA2, and the communication station STA2 energizesthe receiver to receive information from the communication station STA1.

When the ATIM window is ended, the communication stations STA1 and STA2which hold the transmission information energize the back-off timerswhile monitoring the respective states of the media over the randomtime. In the example of FIG. 32, since the timer of the communicationstation STA2 has finished counting first, the communication station STA2first transmits the information to the communication station STA3. Afterthis transmission of the information was ended, the communicationstation STA1 energizes the back-off timer while monitoring again therespective states of the media over the random time, and after the timeris ended, it transmits the packet to the communication station STA2.

In the above-mentioned procedure, a communication station which has notreceived the ATIM packet within the ATIM window or which does not holdinformation de-energizes the transmitter and receiver until the nextTBTT field and it becomes possible to decrease power consumption.

Next, the access contention method of the IEEE802.11 system will bedescribed with reference to FIG. 33. In the above explanation, while wehave described “communication station energizes the back-off timer whilemonitoring the states of the media over the random time”, let us makeadditional explanation to this case.

In the IEEE802.11 system, four kinds of IFS are defined as packet spaces(IFS: Inter Frame Space) extending from the end of theimmediately-preceding packet to the transmission of the next packet. Ofthe four kinds of the inter frame spaces, three inter frame spaces willbe described. As shown in FIG. 33, as the IFS, there are defined SIFS(Short IFS), PIFS (PCF IFS) and DIFS (DCF IFS) in the sequential orderof short inter frame space. According to the IEEE802.11, a CSMA (CarrierSense Multiple Access) is applied as the fundamental media accessprocedure. Accordingly, before the transmission unit transmits someinformation, the communication station energizes the backoff timer overthe random time while monitoring the state of the media. If it isdetermined that the transmission signal does not exist during this timeperiod, then the transmission unit is given a transmission right.

When the communication station transmits the ordinary packet inaccordance with the CSMA procedure (called a DCF: DistributedCoordination Function), after the transmission of some packet has beenended, the state of the media of only the DIFS is monitored. Unless thetransmission signal exists during this time period, then the randombackoff is made. Further, unless the transmission signal exists duringthis time period, the transmission unit is given a transmission right.On the other hand, when a packet such as ACK packet which has anexceptionally large emergency is transmitted, the transmission unit isallowed to transmit the packet after the SIFS packet space. Thus, itbecomes possible to transmit the packet with the large emergency beforethe packet that is to be transmitted in accordance with the ordinaryCSMA procedure. Different kinds of packet spaces IFS are defined forthis reason. Packet transmission contention is prioritized dependingupon whether the IFS is the SIFS or the PIFS or the DIFS. The purpose ofusing the PIFS will be described later on.

Next, the RTS/CTS procedure in the IEEE802.11 will be described withreference to FIGS. 34 and 35. In network under the ad hoc environment,it is generally known that a problem of a hidden terminal arises. As amethodology for solving the most part of this problem, there is known aCSMA/CA based upon the RTS/CTS procedure. The IEEE802.11 also uses thismethodology.

An example of operation in the RTS/CTS procedure will be described withreference to FIG. 34. FIG. 34 shows an example of the case in which someinformation (DATA) is transmitted from a communication station STA0 to acommunication station STA1. Before transmitting actual information, thecommunication station STA0 transmits an RTS (Request To Send) packet tothe communication station STA1 which is an information destinationstation in accordance with the CSMA procedure. When the communicationstation STA1 received this packet, it transmits a CTS (Clear To Send)packet which feeds information indicative of the reception of the RTSpacket back to the communication station STA0 to the communicationstation. When the communication station STA0 which is the transmissionside receives the CTS packet without accident, the communication stationregards that the media is clear and transmits an information (Data)packet immediately. After the communication station STA1 receives thisinformation packet without accident, it returns the ACK packet and thetransmission of one packet is ended.

Actions that will occur in this procedure will be described withreference to FIG. 35. In FIG. 35, it is assumed that a communicationstation STA2 may transmit information to a communication station STA3.Having confirmed by the CSMA procedure that the media is clear during apredetermined period, the communication station STA2 transmits the RTSpacket to the communication station STA3. This packet is also receivedby the neighbor communication station STA1 of the communication stationSTA2. Because the communication station STA1 receives the RTS packet andbecomes aware that the station STA2 intends to transmit someinformation, it recognizes that the media is occupied by the stationSTA2 until the transmission of such information is ended, and it alsobecomes aware of the fact that the media is occupied without monitoringthe media during this time period. This work is called an NAV (NetworkAllocation Vector). The RTS packet and the CTS packet have durations oftime in which the media is occupied in the transaction written thereon.

Returning to the description, having received the RTS packet transmittedfrom the communication station STA2 to the communication station STA3,the communication station STA1 becomes aware of the fact that the mediais placed in the occupied state during a time period designated by theRTS packet, and hence it refrains from transmitting information. On theother hand, the communication station STA3 which received the RTS packetreturns the CTS packet to the communication station to feed informationindicative of the reception of the RTS packet back to the communicationstation STA2. This CTS packet is also received by a neighborcommunication station STA4 of the communication station STA3. Thecommunication station STA4 recognizes by decoding the content of the CTSpacket that information is transmitted from the communication stationSTA2 to the communication station STA3, and it becomes aware of the factthat the media will be occupied during a time period designated by theCTS packet. Hence, it refrains from transmitting information.

When the above-described RTS packet and CTS packet are transmitted andreceived, the transmission is prohibited between “neighboring station ofthe communication station STA2 which is the transmission station” whichcould receive the RTS packet and “neighboring station of thecommunication station STA3 which is the reception station” which couldreceive the CTS packet, whereby information can be transmitted from thecommunication station STA2 to the communication station STA3 and the ACKpacket can be returned without being disturbed by the suddentransmission from the neighboring station.

Next, a band reserve means in the IEEE802.11 system will be describedwith reference to FIG. 36. In the above-mentioned IEEE802.11 systemaccess control, access contention based on the CSMA procedure isexecuted, and hence it is impossible to guarantee and maintain aconstant band. In the IEEE802.11 system, a PCF (Point CoordinationFunction) exists as a mechanism for guaranteeing and maintaining theband. However, the basis of the PCF is polling and it does not operatein the ad hoc mode but it operates only in the infrastructure mode undercontrol of the access point. Specifically, in order to execute theaccess control while the band is being guaranteed, a coordinator such asan access point is required and all controls are carried out by theaccess point.

For reference, operations of the PCF will be described with reference toFIG. 36. In FIG. 36, it is assumed that the communication station STA0is the access point and that the communication stations STA1 and STA2joined in the BSS managed by the access point STA0. Also, it is assumedthat the communication station STA1 transmits information while itguarantees the band.

Having transmitted the beacon, for example, the communication stationSTA0 performs polling to the communication station STA1 at the SIFSspace (CF-Poll in FIG. 36). The communication station STA1 whichreceived the CF-Poll is given a right to transmit data and is therebyallowed to transmit data at the SIFS space. As a result, thecommunication station STA1 transmits the data after the SIFS space. Whenthe communication station STA0 returns the ACK packet for thetransmitted data and one transaction is ended, the communication stationSTA0 again performs polling to the communication station STA1.

FIG. 36 shows also the case in which polling of this time is failed dueto some reason, that is, the state in which the polling packet shown asthe CF-Poll follows the SIFS space. Specifically, when the communicationstation STA0 becomes aware that no information is transmitted from thecommunication station STA1 after the SIFS space elapsed since it hasperformed polling, it regards that the polling is failed and performspolling again after the PIFS space. If this polling is successful, thendata is transmitted from the communication station STA1 and the ACKpacket is returned. Even when the communication station STA2 holds thetransmitted packet during a series of this procedure, since thecommunication station STA0 or STA1 transmits information at the SIFS orPIFS space before the DIFS time space elapses, the right to transmitinformation is never moved to the communication station STA2 and hencethe communication station STA1 to which the polling is performed isconstantly given a priority.

Official Gazette of Japanese laid-open patent application No. 8-98255discloses an example of access control of such wireless communication.

When access control of wireless communication is carried out withoutsuch master control station (access point), as compared with the case inwhich communication is carried out with the master control station,there were various restrictions. To be concrete, the following problemsarise.

Problem 1: Selection of Coordinator

For example, as shown in FIG. 37, let it be assumed that a network isconfigured by the above-mentioned IEEE802.11 system when communicationstations 10 to 17 are located in the scattered state and communicationranges 10 a to 17 a in which the communication stations 10 to 17 candirectly communicate with each other. In such case, if the network isconfigured in the infrastructure mode, then there arises a problem ofhow to select a communication station that should be operated as theaccess point (coordinator). In the IEEE802.11 system, a communicationstation accommodated within the BSS may communicate with only acommunication station which belongs to the same BSS, and the accesspoint is operated as a gateway to other BSS. In order to efficientlymake networking on the whole of the system, there are various argumentssuch as to select which location of the communication station as theaccess point or how to configure again the network when the access pointis de-energized. Although it is desirable that the network could beconfigured without the coordinator, the infrastructure mode of theIEEE802.11 system cannot meet with such requirements.

Problem 2: Disagreement of Achievable Area

In the ad hoc mode of the IEEE802.11 system, although the network can beconfigured without the coordinator, it is assumed that the IBSS isconstructed by a plurality of communication stations located at thesurrounding areas. For example, as shown in FIG. 37, it is assumed thatthe communication stations 10, 11, 12, 13 (STA0, STA1, STA2, STA3) areaccommodated within the same IBSS. Then, although the communicationstation 11 (STA1) can communicate with the communication stations 10,12, 13 (STA0, STA2, STA3), the communication station 10 (STA0) cannotdirectly communicate with the communication station 12 (STA2). In suchcase, according to the beacon transmission procedure of the IEEE802.11system, it is frequently observed that the communication station 10(STA0) and the communication station 12 (STA2) transmit the beacons atthe same time, and at that time, the communication station 11 (STA1)becomes unable to receive a beacon, which causes a problem.

Further, as shown in FIG. 37, for example, let it be assumed that thecommunication stations 15, 16, 17 (STA5, STA6, STA7) constitute an IBSS(IBSS-A) and that the communication stations 10, 11, 12, 13 (STA0, STA1,STA3, STA3) constitute an IBSS (IBSS-B). At that time, since the twoIBSSes are operating completely independently, an interference problemdoes not arise between the two IBSSes. Here, let it be considered thecase in which a new communication station 14 (STA4) appears on thenetwork. Then, the communication station 14 (STA4) is able to receiveboth signals from the IBSS-A and the IBSS-B. When the two IBSSes arecoupled together, although the communication station STA4 can enter bothof the IBSS-A and the IBSS-B, the IBSS-A is operated in accordance withthe rule of the IBSS-A and the IBSS-B is operated in accordance with therule of the IBSS-B. Then, there is a possibility that collision of thebeacons and collision of the ATIM packets will occur, which also raisesa problem.

Problem 3: Method of Realizing Power Save Mode

In the ad hoc mode, the power save mode can be realized by transmittingthe ATIM packets with each other within the ATIM window according to therandom access. When information to be transmitted is a small amount ofinformation such as bits, an overhead required by the ATIM packetsincreases, and a methodology in which the ATIM packets are to beexchanged according to the random access is very inefficient.

Problem 4: Band Reserve in Network Without Coordinator

Also, according to the IEEE802.11 system, in the ad hoc mode, amechanism for carrying out band reserve does not exist, and hence thereis no method but to constantly follow the operation of the CSMAprocedure.

Problem 5: Incompleteness of RTS/CTS Procedure

In the RTS/CTS procedure of the IEEE802.11 system, not only acommunication station which received the CTS packet but also acommunication station which received the RTS packet is prohibited fromtransmitting information. However, in the case shown in FIG. 35, thestation that is prohibited from transmitting information is only thecommunication station STA4 and the communication station STA1 does notaffect “transmission of DATA from the communication station STA2 to thecommunication station STA3”. In the RTS/CTS procedure, to prohibit thecommunication station which received the RTS packet from transmittinginformation requires a large margin to the safety side and this is oneof the factors which degrade a system throughput.

Problem 6: Considerations on Separation of BBSES by TDMA

In the scenario described in the above-mentioned Problem 2 (in FIG. 37,the communication stations STA5, STA6, STA7 constitute the IBSS (IBSS-A)and the communication stations STA0, STA1, STA2, STA3 constitute theIBSS (IBSS-B)), as a method for solving the problem which arises whenthe communication station STA4 appears to couple both of the IBSSes,there exists a method for separating the IBSS-A and the IBSS-B by a TDMA(Time Division Multiple Access: time division multiple access) system.An example of this case is shown in FIG. 38. This is a method used in anARIB STD-T70 (HiSWANa) system and the like. A time zone that isexclusively used for a sub-network is constructed in a frame of someBBS. However, according to this method, spatial recycling of resourcesis aborted and hence utilization ratio is decreased considerably, whichalso causes a problem.

In view of the aforesaid aspects, it is an object of the presentinvention to provide excellent wireless communication system, wirelesscommunication apparatus and wireless communication method and computerprogram in which the problems arising when a wireless system such as awireless LAN is constructed as a decentralized distributed type networkwithout control and controlled relationship such as a master station andslave stations can be solved.

Other object of the present invention is to provide excellent wirelesscommunication system, wireless communication apparatus and wirelesscommunication method and computer program in which data can betransmitted while collisions are being avoided in a decentralizeddistributed type network.

A further object of the present invention is to provide excellentwireless communication system, wireless communication apparatus andwireless communication method and computer program in which collisionsof beacons can be suitably avoided among a plurality of communicationstations in a network configured when communication stations transmitbeacons with each other.

Yet a further object of the present invention is to provide excellentwireless communication system, wireless communication apparatus andwireless communication method and computer program in which adecentralized distributed type wireless network can be suitably formedwhile collisions of beacons that communication stations transmitted witheach other can be avoided.

DISCLOSURE OF THE INVENTION

The present invention is made in view of the aforesaid aspect. Accordingto an aspect of the present invention, there is disclosed in connectionwith a wireless communication system composed of a plurality ofcommunication stations without relationship of control station andcontrolled stations, wherein respective communication stations transmitbeacons in which information concerning a network is written with eachother to construct the network.

However, “system” refers to something of logical set of a plurality ofapparatus (or function modules that can realize specific function)regardless of whether each apparatus or function module is accommodatedwithin a single housing or not.

Under the decentralized distributed type communication environment, eachcommunication station lets other neighbor (within a communication range)communication station become aware of its existence by transmittingbeacon information to other neighbor communication station at apredetermined time space and also lets other communication stationbecome aware of the network configuration. Also, the communicationstation executes scan operation on each channel and detects by receivinga beacon signal whether it joined the communication range of theadjacent station. Further, the communication station can recognize thenetwork configuration by deciphering information written on the beacon.

Also, each communication station transmits neighbor apparatusinformation concerning beacon transmission timing contained in thebeacon signal. In such case, the communication station can obtain notonly network information of the adjacent station from which thecommunication station can directly receive the beacon but also beaconinformation of the next station of the adjacent station from which thelocal station cannot receive the beacon but the adjacent station canreceive the beacon, that is, a hidden terminal.

In such decentralized distributed type network, a new communicationstation which joins the network attempts to execute scan operation, thatis, to continuously receive a signal during a time period longer than asuperframe length to confirm the presence of the beacon transmitted fromthe neighboring station. If the communication station cannot receive thebeacon from the neighboring station in this process, then thecommunication station sets proper beacon transmission timing. On theother hand, if the communication station can receive the beacontransmitted from the neighboring station, then the communication stationsets timing at which any one of existing stations does not transmit thebeacon to the beacon transmission timing of the local station withreference to neighbor apparatus information described in each receivedbeacon.

Here, in the wireless communication network according to the presentinvention, each communication station obtains a traffic priority useperiod as it transmits the beacon. Then, each communication station maytransmit a regular beacon only once at the above-described predeterminedtime space and may be allowed to transmit more than one auxiliary beaconcomposed of signals similar to the regular beacon.

Also, according to a second aspect of the present invention, in acomputer program written in the computer readable format such thatprocessing for carrying out wireless communication operation under thedecentralized distributed type communication environment configured whena specific control station is not located and respective communicationstations transmit beacons with information concerning a network writtenthereon with each other at a predetermined time space may be executed, acomputer program is comprised of a beacon signal generating step forgenerating a beacon signal with information concerning the local stationwritten thereon, a beacon signal analyzing step for analyzing a beaconsignal received from a neighboring station and a timing control step forcontrolling beacon transmission timing.

The computer program according to the second aspect of the presentinvention is obtained by defining a computer program written in thecomputer readable format so that predetermined processing may beexecuted on the computer system. In other words, when the computerprogram according to the second aspect of the present invention isinstalled on the computer system, cooperative action is demonstrated onthe computer system and thereby the computer system is operated as awireless communication apparatus. A plurality of wireless communicationapparatus may be activated to construct a wireless network with similaraction and effects to those of the wireless communication systemaccording to the first aspect of the present invention.

According to the present invention, in a decentralized distributed typenetwork having a control station/controlled station relationship such asa master station and slave stations, it is possible to provide excellentwireless communication system, wireless communication apparatus andwireless communication method and computer program in which data can betransmitted while collisions of beacons can be avoided.

Also, according to the present invention, in a network configured whencommunication stations transmit beacons with each other, it is possibleto provide excellent wireless communication system, wirelesscommunication apparatus and wireless communication method and computerprogram in which collisions of beacons among a plurality ofcommunication stations can be avoided suitably.

Also, according to the present invention, it is possible to provideexcellent wireless communication system, wireless communicationapparatus and wireless communication method and computer program inwhich a decentralized distributed type wireless network can be suitablyformed while collisions of beacons that respective communicationstations transmit with each other can be avoided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram showing an example in whichcommunication apparatus are located according to an embodiment of thepresent invention;

FIG. 2 is a block diagram showing an example of an arrangement of acommunication apparatus according to an embodiment of the presentinvention;

FIG. 3 is a timing chart showing an example of a wireless communicationsystem according to an embodiment of the present invention;

FIG. 4 is a timing chart showing an example of timing at which beaconsare transmitted according to an embodiment of the present invention;

FIG. 5 is an explanatory diagram showing part of beacon descriptioninformation according to an embodiment of the present invention;

FIG. 6 is an explanatory diagram showing an example of NOBI and NBAIprocessing procedures according to an embodiment of the presentinvention;

FIG. 7 is an explanatory diagram showing an example of the manner inwhich a transmission prohibit interval is defined according to anembodiment of the present invention;

FIG. 8 is an explanatory diagram showing a first example of a beaconcollision scenario according to an embodiment of the present invention;

FIG. 9 is an explanatory diagram showing a second example of a beaconcollision scenario according to an embodiment of the present invention;

FIG. 10 is an explanatory diagram showing a beacon transmission offsetaccording to an embodiment of the present invention;

FIG. 11 is an explanatory diagram showing part of beacon descriptioninformation according to an embodiment of the present invention;

FIG. 12 is a block diagram showing an example of an M-sequencegenerating circuit according to an embodiment of the present invention;

FIG. 13 is a flowchart showing an example of timing control processingaccording to an embodiment of the present invention;

FIG. 14 is an explanatory diagram showing an example of a manner ofdetermining a packet space according to an embodiment of the presentinvention;

FIG. 15 is an explanatory diagram showing an example of a transmissionprioritized interval according to an embodiment of the presentinvention;

FIG. 16 is an explanatory diagram showing the transmission prioritizedinterval and a conflict transmission interval according to an embodimentof the present invention;

FIG. 17 is an explanatory diagram showing an example of a packet formataccording to an embodiment of the present invention;

FIG. 18 is an explanatory diagram showing an example of a beacon signalformat according to an embodiment of the present invention;

FIG. 19 is a timing chart showing an example (example 1) of thecommunication state at a communication station according to anembodiment of the present invention;

FIG. 20 is a timing chart showing an example (example 2) of thecommunication state at a communication station according to anembodiment of the present invention;

FIG. 21 is an explanatory diagram showing an example of a manner ofdistributing a time-axis resource according to an embodiment of thepresent invention;

FIG. 22 is an explanatory diagram showing an example of informationwhich is used to determine beacon transmission timing according to anembodiment of the present invention;

FIG. 23 is an explanatory diagram showing an example of band reserveprocessing according to an embodiment of the present invention;

FIG. 24 is an explanatory diagram showing an example of the manner inwhich a quiet packet is used according to an embodiment of the presentinvention;

FIG. 25 is an explanatory diagram showing an example of an arrangementof a quiet packet according to an embodiment of the present invention;

FIG. 26 is an explanatory diagram showing an example of an arrangementof a PHY frame according to an embodiment of the present invention;

FIG. 27 is an explanatory diagram showing an example (example 1) ofmedia scan according to an embodiment of the present invention;

FIG. 28 is an explanatory diagram showing an example of the manner inwhich data is transmitted a plurality of times according to anembodiment of the present invention;

FIG. 29 is an explanatory diagram showing an example (example 2) ofmedia scan according to an embodiment of the present invention;

FIG. 30 is an explanatory diagram showing an example (infrastructuremode) of a conventional wireless communication system;

FIG. 31 is an explanatory diagram showing an example (ad hoc mode) of aconventional wireless communication system;

FIG. 32 is an explanatory diagram showing an example of a signaltransmission procedure in the ad hoc mode according to the prior art;

FIG. 33 is an explanatory diagram showing an example of a packet spacein the conventional wireless communication system;

FIG. 34 is an explanatory diagram showing an example of a CSMA/CAprocedure in the conventional wireless communication system;

FIG. 35 is an explanatory diagram showing an example of CSMA/CAoperation in the conventional wireless communication system;

FIG. 36 is an explanatory diagram showing an example of band reservetransmission in the conventional wireless communication system;

FIG. 37 is an explanatory diagram showing an example of thecommunication state in the conventional wireless communication system;and

FIG. 38 is an explanatory diagram showing an example of an arrangementof a sub-slot in the conventional wireless communication system.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment according to the present invention will be described belowwith reference to FIGS. 1 to 29.

A propagation line of communication assumed in this embodiment of thepresent invention is wireless and it is also assumed that a network isconstructed among a plurality of devices by using a single transmissionmedium (when a link is not separated by a frequency channel). This willapply for the case in which a plurality of frequency channels exists astransmission mediums) as well. Communication assumed in this embodimentis a store and forward type traffic and hence information is transferredat the packet unit. Also, processing at each communication station whichwill be described below is fundamentally processing executed by allcommunication stations joined the network. However, depending on thecases, all communication stations comprising the network do not alwaysexecute the processing which will be described below.

FIG. 1 shows an example of the manner in which communication apparatuscomprising the wireless communication system according to an embodimentof the present invention are located. In this wireless communicationsystem, a specific control station is not located and respectivecommunication stations are operated in a decentralized distributedfashion to form a so-called ad hoc network. This sheet of drawing showsthe manner in which communication apparatus #0 to #6 are distributed onthe same space.

Also, in this sheet of drawing, communication ranges of the respectivecommunication apparatus are shown by broken lines. Communicationapparatus can communicate with other communication apparatus locatedwithin such communication range and these communication ranges aredefined as the ranges in which a signal transmitted from the localstation interfere with signals transmitted from other communicationapparatus. Specifically, the communication apparatus #0 is locatedwithin the range in which it can communicate with neighbor communicationapparatus #1, #4, the communication apparatus #1 is located within therange in which it can communicate with the neighbor communicationapparatus #0, #2, #4, the communication apparatus #2 is located withinthe range in which it can communicate with the neighbor communicationapparatus #1, #3, #6, the communication apparatus #3 is located withinthe range in which it can communicate with the neighbor communicationapparatus #2, the communication apparatus #4 is located within the rangein which it can communicate with the neighbor communication apparatus#0, #1, #5, the communication apparatus #5 is located within the rangein which it can communicate with the neighbor communication apparatus#4, and the communication apparatus #6 is located within the range inwhich it can communicate with the neighbor communication apparatus #2.

When communication is made between certain specific communicationapparatus, a communication apparatus which can receive information fromone communication apparatus of the apparatus being called but whichcannot receive information from other communication apparatus, that is,“hidden terminal” exists.

FIG. 2 is a block diagram schematically showing function and arrangementof a wireless communication apparatus which is operated as acommunication station in the wireless network according to theembodiment of the present invention. Under a decentralized distributedtype communication environment in which a control station is notlocated, the illustrated wireless communication apparatus can form anetwork by effectively carrying out channel access within the samewireless system while collisions are being avoided.

As illustrated, a wireless communication apparatus 100 is composed of aninterface 101, a data buffer 102, a central control unit 103, a beacongenerating unit 104, a wireless transmission unit 106, a timing controlunit 107, an antenna 109, a wireless reception unit 110, a beaconanalyzing unit 112 and an information storage unit 113.

The interface 101 exchanges a variety of information between it and anexternal device (for example, a personal computer (not shown), etc.)connected to this wireless communication apparatus 100.

The data buffer 102 is used to temporarily store data transmitted from adevice connected through the interface 101 or data received through awireless transmission line before such data is transmitted via theinterface 101.

The central control unit 103 controls transmission and reception of aseries of information in the wireless communication apparatus 100 andperforms access control of a transmission line in a centralized fashion.The central control unit 103 performs operation control such ascollision avoidance when beacons collide with each other.

The beacon generating unit 104 generates beacon signals that areperiodically exchanged between it and the neighbor wirelesscommunication apparatus. In order for the wireless communicationapparatus 100 to use the wireless network, there should be stipulatedthe position at which its own beacon is transmitted and the position atwhich it receives a beacon from the neighboring station. Thisinformation is stored in the information storage unit 113 and istransmitted to neighboring wireless communication apparatus in the formin which it is written in the beacon signal. An arrangement of thebeacon signal will be described later on. Since the wirelesscommunication apparatus 100 transmits a beacon at the beginning of atransmission frame period, a transmission frame period in the channelused by the wireless communication apparatus 100 is defined by a beaconspace.

The wireless transmission unit 106 carries out predetermined modulationprocessing in order to transmit data temporarily stored in the databuffer 102 and a beacon signal via radio waves. Also, the wirelessreception unit 110 receives information and a beacon signal transmittedfrom other wireless communication apparatus at a predetermined time.

Various communication systems applicable to wireless LAN, for example,which is suitable for relatively short-distance communication, can beapplied to the wireless transmission and reception system in thewireless transmission unit 106 and the wireless reception unit 110. Tobe concrete, it is possible to use a UWB (Ultra Wide Band) system, anOFDM (orthogonal Frequency Division Multiplexing: orthogonal frequencydivision multiplexing) system, a CDMA (Code Division Multiple Access:code division multiple access) system and the like.

The antenna 109 transmits a signal to other wireless communicationapparatus through a predetermined frequency channel or collects signalstransmitted from other wireless communication apparatus. In thisembodiment, the communication apparatus includes a single antenna and isunable to receive and transmit signals concurrently.

The timing control unit 107 controls timing at which a wireless signalshould be transmitted and received. For example, the timing control unitcontrols its own beacon transmission timing at the beginning of thetransmission frame period, timing at which it receives a beacon fromother communication apparatus, timing at which it transmits and receivesdata between it and other communication apparatus and a scan operationperiod, etc.

The beacon analyzing unit 112 analyzes the existence of the neighborwireless communication apparatus by analyzing the beacon signal receivedfrom the adjacent station, For example, information such as the beaconreception timing of the adjacent station and the neighboring stationbeacon reception timing is stored in the information storage unit 113 asneighbor apparatus information.

The information storage unit 113 stores therein an execution procedurecommand (program in which a collision avoidance processing procedure andthe like are described) such as a series of access control operationsexecuted by the central control unit 103 and neighbor apparatusinformation obtained from the analyzed result of the received beacon.

In the decentralized distributed type network according to thisembodiment, each communication station lets other neighbor (that is,within a communication range) station become aware of its existence bytransmitting beacon information at a predetermined time space on apredetermined channel and also informs other communication station ofthe network arrangement. The beacon transmission period is defined as“superframe” (Super frame) of which duration is 80 milliseconds, forexample.

A new communication station that joins the network can recognize that itentered the communication range by receiving the beacon signal from theneighboring station through scan operation and it can recognize thenetwork arrangement by deciphering information described in the beacon.Then, the new communication station sets its own beacon transmissiontiming to timing at which a beacon is not transmitted from theneighboring station in gentle synchronism with the beacon receptiontiming.

Next, FIG. 17 shows an example of a packet format according to thisembodiment. A preamble composed of a unique word is added to thebeginning of the packet in order to demonstrate the existence of thepacket. In a heading area transmitted immediately after the preamble,there are stored attribute of this packet, length, transmission powerand a payload portion transmission rate if PHY is in themulti-transmission rate mode. The heading area decreases itstransmission rate so that a predetermined SNR may decrease several [dB]as compared with that of the payload portion. This heading area isdifferent from a so-called MAC header and the MAC header is contained inthe Payload portion. The payload portion is the portion depicted as PSDU(PHY Service Data Unit) in FIG. 17 and in which there is stored a bearerbit string containing a control signal and information. The PSDU iscomposed of the MAC header and an MSDU (MAC Service Data Unit), and theMSDU portion stores therein a data string transferred from a high-orderlayer.

In the following description, in order to describe the present inventionconcretely, it is assumed that duration of the preamble is 8 [μsec], abit rate of the payload portion is 100 Mbps upon transmission and thatthe heading area is composed of 3 bytes and transmitted at 12 [Mbps].Specifically, when one PSDU is transmitted and received, there occurs anoverhead of 10 [μsec] (=preamble 8 [μsec]+heading 2 [μsec]).

A fundamental access procedure in this embodiment is the same CSMA/CA asthat of the prior art, and information is transmitted after it wasconfirmed that the media is clear before information is transmitted.

Beacon Transmission Procedure:

First, a beacon transmission procedure of each communication stationaccording to this embodiment will be described with reference to FIG. 3.Each communication station that joined the network transmits a beaconperiodically in order to let the neighboring station become aware of theexistence of the communication station. Here, the period is assumed tobe 80 [msec] and let us describe the present invention with reference tothe case in which the beacon is transmitted at every 80 [msec]. However,the above period is not always limited to 80 [msec].

Assuming that information transmitted by the beacon is 100 bytes, then atime required by transmission becomes 18 [μsec]. Since the beacon istransmitted once at 80 [msec], a beacon media occupying rate of onecommunication station is as sufficiently small as 1/4444. Although itseem to be useless that a beacon is transmitted even when thetransmission signal does not arrive at the station, the transmissiontime rate is as sufficiently small as 1/4444, and this problem does notbecome serious.

The respective communication stations are gently synchronized with eachother while receiving and confirming the beacons transmitted from theneighbor communication stations. When a new communication station joinedthe network, the new communication station sets beacon transmissiontiming of the local station to timing at which a beacon is nottransmitted from the neighbor communication station. An example thereofwill be described below.

When the neighbor communication station does not exist, as shown in FIG.3A, a communication station [number 01] can begin to transmit a beaconat proper timing. B01 shows transmission position (timing) of a beacontransmitted from the communication station [number 01]. The beacontransmission period is defined as superframe (Superframe) and a beaconspace is 80 [msec]. Also in FIGS. 3B, 3C, 3D, positions depicted bycommunication station numbers added to B show communication timing.

After that, the newly-joined communication station starts transmitting abeacon in substantially the center of a time zone with the longestbeacon space in the range in which it can receive the beacon such thatits beacon may not collide with beacons transmitted from othercommunication stations which were already located within the superframe.For example, when a new communication station [number 02] appears in thebeacon transmission state shown in FIG. 3A, it starts transmitting abeacon at the middle timing of the beacon space of the communicationstation [number 01] while it is recognizing the existence of thecommunication station 01.

After that, a new communication station which joined the communicationrange sets its own beacon transmission timing so that it may not collidewith the layout of the existing beacons. At that time, since eachcommunication station obtains a prioritized use area (TPP) immediatelyafter it has transmitted the beacon (as will be described later on), itis preferable that beacon transmission timing of each communicationstation should be equally dispersed within the transmission frame periodrather than crowded from a transmission efficiency standpoint.Accordingly, in this embodiment, the new communication station can starttransmitting the beacon in substantially the middle of the time zonewith the longest beacon space within the range in which it can receivethe beacon from other communication station.

Further, when a new communication station [number 03] appears in thestate shown in FIG. 3B, it starts transmitting a beacon in the middletiming of the beacon space while it is confirming the existence of thecommunication state [number 01] and the communication station [number02]. After that, according to a similar algorithm, as shown in FIGS. 3Cand 3D, the beacon space is narrowed as a neighbor communication stationoccurs. However, as the beacon space is narrowed in this manner, theband (transmission frame period) is occupied by the beacons so that aminimum beacon space should be stipulated such that the band may not befilled with the beacons. For example, when the beacon space isstipulated as the minimum beacon space Bmin=625 [μsec], only 128communications can be accommodated within the range in which radio wavescan be received and transmitted at maximum.

FIG. 4 shows an example of an arrangement of beacon transmission timingwhich can be located within the superframe. However, the illustratedexample expresses the elapse of time in the superframe of 80milliseconds as a clock of which hand is rotated in the clockwisedirection on the circular ring.

In the example shown in FIG. 4, 16 positions 0 to F from ranging 0 to Fin total are constructed as times at which beacons can be transmitted,that is, “slots” in which beacon transmission timing can be located. Ashad already been described with reference to FIG. 3, let it be assumedthat beacons are located in accordance with the algorithm in which thebeacon transmission timing of the newly-joined station is sequentiallyset to at substantially the middle timing of the beacon space set by theexisting communication station. When Bmin is stipulated as 5milliseconds, only 16 beacons can be located at maximum per superframe.That is, more than 16 communication stations cannot join the network.

Although not shown explicitly in FIGS. 3 and 4, each beacon istransmitted at a time which is intentionally displaced from a TBTT(Target Beacon Transmission Time) by a small time offset. This will bereferred to as “TBTT offset”. In this embodiment, a TBTT offset value isdetermined by a pseudorandom number. This pseudorandom number isdetermined by a pseudorandom sequence TOIS (TBTT Offset IndicationSequence) that is uniquely determined and the TOIS is updated at everysuperframe period.

With the TBTT offset, even when two communication stations have beacontransmission timings located at the same slot on the superframe, actualbeacon transmission times can be displaced. Hence, even when the beaconscollide with each other in a certain superframe period, the respectivecommunication stations can transmit and receive their beacons in anothersuperframe period (or the neighbor communication station can receive thebeacons from both of the above communication stations) so that thecommunication station can recognize that the beacon of the local stationcollided with other beacons. The communication station includes the TOISset at every superframe period in the beacon information and transmitsthis resultant beacon information to the neighboring station (which willbe described later on).

Also, according to this embodiment, each communication station is oughtto carry out reception operation before and after the beacon transmittedfrom the local station when it does not transmit and receive data. Also,even when each communication station does not transmit and receive data,it is ought to carry out scan operation by continuously energizing thereceiver over one superframe once per several seconds to thereby confirmwhether or not the presence of the beacon from the neighboring stationis changed or whether or not the TBTT of each neighboring station isdisplaced. Then, if it is determined that the TBTT is displaced, then atarget beacon transmission time in which a displacement within −Bmin/2milliseconds is stipulated as TBTT with reference to a TBTT grouprecognized by the local station is defined as “advanced target beacontransmission time” and a target beacon transmission time in which adisplacement within +Bmin/2 milliseconds is stipulated as TBTT isdefined as “delayed target beacon transmission time”, and a time iscorrected in accordance with the most delayed TBTT.

NBOI Field:

As one of information transmitted by a beacon, FIG. 5 shows an exampleof the manner in which a Neighboring Beacon offset Information (NBOI)field is described. The position of the beacon that can be received bythe local station (reception time) is written on the NBOI field by therelative position (relative time) from the position (transmission time)of the beacon of the local station in the form of a bit map. The exampleshown in FIG. 5 describes the case in which only 16 kinds of beacontransmission positions can exist at the minimum space Bmin=5 [msec], byway of example, and hence the NBOI field length is 16 bits but it maynot always be limited to 16 bits.

The example of FIG. 5 shows an example of the NBOI field indicative ofthe message that “the communication station (number 0) in FIG. 4 canreceive the beacons from the communication station [number 1] and thecommunication station [number 9]”. With respect to the bitscorresponding to the relative positions of the beacons that can bereceived, the relative position at which the beacon is received isdepicted by a mark and the relative position at which the beacon is notreceived is depicted by a space. In the example of FIG. 5, 0th bit,first bit and ninth bit are depicted by the marks. The mark on the 0thbit indicates that the local station transmitted the beacon, and themark on the first bit indicates that the beacon is received at timingdelayed from the TBTT field of the beacon by a delay amount of Bmin*1.Similarly, the mark on the ninth bit indicates that the beacon isreceived at timing delayed from the TBTT field of the beacon by a delayamount of Bmin*9.

Although the details will be described later on, bits corresponding totiming at which the beacon is not received may be depicted by a mark forother purpose such as when an auxiliary beacon is transmitted.

NBAI Field:

Also, similarly to the NBOI field, a Neighboring Beacon ActivityInformation (NBAI) field is defined as one of information similarlytransmitted by the beacon. The NBAI field describes the position(reception time) of the beacon that is actually received by the localstation based upon the relative position of the beacon from the localstation in the form of a bit map. Specifically, the NBAI field indicatesthat the local station is set to the active state in which it is able toreceive a beacon.

Further, based upon the information of the NBOI field and the NBAIfield, it is possible to provide information in which the local stationreceives a beacon at the beacon position within the superframe.Specifically, based on the NBOI field and the NBAI field contained inthe beacon, the following two-bit information is transmitted to eachcommunication station.

NBAI NBOI Description 0 0 BEACON IS NOT RECOGNIZED AT CORRESPONDING TIME0 1 BEACON IS RECOGNIZED AT CORRESPONDING TIME 1 0 COMMUNISCATIONSTATION IS SET TO ACTIVE STATE AT CORRESPONDING TIME 1 1 COMMUNICATIONSTATION IS RECEIVING BEACON AT CORRESPONDING TIME

Or-Processing of NBOI/NBAI:

FIG. 6 shows the manner in which a communication station A which joinedthe network sets the TBTT field of the local station based upon the NBOIfield of each beacon obtained from the beacons received from theneighboring station by scan operation.

Let it be assumed that the communication station could receive thebeacons from the stations 0 to 2 during the superframe by scanoperation.

The beacon reception time of the neighboring station is treated as therelative position relative to the regular beacon of the local stationand the NBOI field writes this beacon reception time in the bit mapformat (mentioned hereinbefore). Accordingly, the communication stationA shifts the NBOI fields of the three beacons received from theneighboring stations and arranges the bit corresponding positions on thetime axis, whereafter this communication station calculates a sum ofNBOI bits of each timing to thereby synthesize the NBOI bits forreference. A procedure thereof will be described concretely. A beacon 1is received with a delay of three slots with reference to thetransmission timing of a beacon 0. The communication station stores thisinformation in a memory and so on. Then, three slots of the NBOI fieldcontained in the beacon 1 are shifted to the beginning and thisinformation is stored in a suitable means such as a memory (second rowin FIG. 6)). Similar processing is effected on the beacon 2 (third rowin FIG. 6).

A sequence obtained after the NBOI fields of the neighboring stationswere synthesized and referred to is “1101, 0001, 0100, 1000” shown by“OR of NBOIs” in FIG. 6. “1” indicates the relative position of timingat which the TBTT field was already set within the superframe, and “0”indicates the relative position of timing at which the TBTT field is notyet set. In this sequence, the place in which the space (zero) becomesthe longest run-length becomes a nominated place where a new beacon islocated. In the example shown in FIG. 6, the longest run-length is 3 andtwo nominated places exist. Then, the communication station A determines15th-bit timing as the TBTT field of the regular beacon of the localstation.

The communication station A sets the 15th-bit time as the TBTT field ofthe regular beacon of the local station (that is, the leading portion ofthe superframe of the local station) and starts transmitting the beacon.At that time, the NBOI field in which the communication station Atransmits the beacon writes each reception time of the beacons of thecommunication stations 0 to 2 which can receive the beacons in the bitmap format in which the bit positions corresponding to the relativepositions from the transmission time of the regular beacon of the localstation are marked. This is shown as “NBOI for TX (1 Beacon TX) in FIG.10.

When the communication station A transmits the auxiliary beacon for thepurpose of obtaining a prioritized transmission right and the like,after that, this communication station searches the longest run-lengthof the space (zero) of the sequence shown by “OR of NBOIs” in which theNBOI field of the neighboring station is synthesized and sets atransmission time of the auxiliary beacon to the thus searched space.The example of FIG. 10 assumes the case in which the communicationstation transmits two auxiliary beacons, and the transmission timing ofthe auxiliary beacons is set to the times of 6th-bit and 11th-bitspaces. In this case, in the NBOI field during which the communicationstation A transmits the beacon, in addition to the relative positions ofthe regular beacon of the local station and the beacons received fromthe neighboring stations, the positions at which the local stationtransmits the auxiliary beacon (relative position to the regular beacon)also are marked and presented as shown by “NBOI for Tx (3 Beacon Tx)”.

When each communication station sets the beacon transmission timing TBTTof the local station by the above-mentioned processing procedure andtransmits the beacon, under the condition in which each communicationstation is in the stationary state and the range in which radio wavesreach is not fluctuated, it is possible to avoid the beacons fromcolliding with each other. Also, the auxiliary beacon (or a plurality ofsignals similar to the beacon) is transmitted within the superframe inresponse to a degree of priority of transmission data, whereby resourcescan be assigned with a priority and Qos communication can be provided.Also, since each communication station can independently understandsaturation of the system with reference to the number of beacons (NBOIfields) received from the neighboring station, the present invention,even though it is the distributed control system, can accommodateprioritized traffic while considering saturation of the system at everycommunication station. Further, since each communication station studiesthe NBOI field of the received beacon so that the beacon transmissiontimes may not collide with each other, even when a plurality ofcommunication stations accommodates the prioritized traffic, it ispossible to avoid the beacon transmission times from frequentlycolliding with each other. As described above, when the newcommunication station joins the network, the sum of the NBOI fieldsobtained from the beacons received from the respective communicationstations is calculated so that the center of the interval in which therun-length of the space becomes longest is determined as the beacontransmission timing.

While the above description is the example in which the sum of the NBOIfields are calculated by OR, a sum (OR) of the NBAI fields is calculatedby a similar procedure, whereby a beacon is not transmitted in thebeacon transmission time of the marked timing under control.

Specifically, when the communication station transmits some information,the beacon transmitted from the neighbor communication station isreceived and a sum (OR) of the NBAI fields obtained from the beaconsreceived from the respective communication stations is calculated sothat the beacon is not transmitted in the beacon transmission time ofthe marked timing.

FIG. 7 shows the processing executed in that case in which the NBAIfield is formed of 8 bits and in which 0th-bit, 4th-bit and 6th-bit aremarked after a sum of NBAI fields of respective received beacons wascalculated (OR), by way of example. The 0th-bit is the beacon of thelocal station and hence the addition processing is not carried out.Since the 4th-bit is marked, at the time T4 which is the beacontransmission time of the 4th-bit, the transmission permission flag ofthe local station is not raised. Also, this applies for the 6th-bit aswell and hence at the corresponding time T6, the transmission permissionflag of the local station is not raised and the transmission is notcarried out. Thus, when a certain communication station wants to receivea beacon from a certain communication station, the transmission stationcan be prohibited from disturbing this reception and it becomes possibleto transmit and receive information with high reliability.

FIRST EXAMPLE OF BEACON COLLISION SCENARIO

An example of the manner in which information obtained from the NBOIfield is used will be described with reference to FIG. 8. The left-handsides of FIGS. 8A to 8C show the states in which the communicationstations are located, and the right-hand sides thereof show examples inwhich beacons are transmitted from the respective stations,respectively.

FIG. 8A shows the case in which only a communication station 10 (STA0)exists to transmit a beacon B0. At that time, since the communicationstation 10 attempts to receive a beacon but failed to receive thebeacon, proper beacon transmission timing can be set and transmission ofthe beacon B0 can be started in response to the arrival of this timing.Here, the beacon is transmitted at the space of 80 [msec]. At that time,all bits of the NBOI field of the beacon transmitted from thecommunication station 10 are 0.

FIG. 8B shows the case in which a communication station 11 (STA1) joinedthe communication range of the communication station 10 later on. Whenthe communication station 11 attempts to receive a beacon, it receivesthe beacon B0 of the communication station 10. Further, since all bitsof the NBOI field of the beacon B0 of the communication station 10 areall 0 except the bits indicating the transmission timing of the localstation, the beacon transmission timing is set to substantially thecenter of the beacon space of the communication station 10 in accordancewith the above-mentioned step 1. In the NBOI field of a beacon B1transmitted from the communication station 11, the bit indicative of thetransmission timing of the local station and the bit indicative of thereception timing of the beacon from the communication station 10 are setto 1 and other bits are set to 0. Also, when the communication station10 also becomes aware of the beacon from the communication station 11,it sets the corresponding NBOI field to 1.

Further, FIG. 8C shows the case in which a communication station 12(STA2) joined the communication range of the communication station 11later on. In the example of FIG. 8, the communication station 10 servesas a hidden terminal for the communication station 12. For this reason,since the communication station 12 cannot recognize that thecommunication station 11 is receiving the beacon from the communicationstation 10, there is a possibility that this communication station willtransmit the beacon at the same timing as that of the communicationstation 10 so that their beacons will collide with each other. The NBOIfield is used to avoid this phenomenon. When the communication station12 attempts to receive the beacon, it receives the beacon B1 from thecommunication station 11. Further, in the NBOI field of the beacon B1from the communication station 11, in addition to the bit indicative ofthe transmission timing of the local station, 1 is also set to the bitwhich indicates timing at which the communication station 10 istransmitting the beacon. For this reason, even when the communicationstation 12 cannot directly receive the beacon B0 transmitted from thecommunication station 10, it recognizes the timing at which thecommunication station 10 transmits the beacon B0 and will not transmitthe beacon at this timing. Accordingly, at that time, the communicationstation 12 sets the beacon transmission timing to substantially themiddle between the space of the beacon transmitted from thecommunication station 10 and the space of the beacon transmitted fromthe communication station 11. Of course, in the NBOI field of the beaconB2 transmitted from the communication station 12, bits indicative of thebeacon transmission timings of the communication stations 12 and 11 areset to 1.

The NBOI field in which the beacons are transmitted at the same timingas that of the communication station 10 to cause the beacons to collidewith each other is used to avoid this phenomenon. That is, the NBOIfield is used to avoid the occurrence of the beacon collision scenario(first example) shown on the right-hand side of FIG. 8C.

As described above, in the wireless communication system according tothis embodiment, each communication station transmits the beaconinformation to other communication station so that other communicationstation can recognize the existence of the local station and can alsorecognize the network arrangement. The new communication station, joinedto the network, receives the beacon signal so that it can detect that itentered the communication range. At the same time, such newcommunication station deciphers the information written in the beaconand can transmit the beacon while avoiding its beacon signal fromcolliding with the existing beacon signal and thereby a new network canbe configured.

SECOND EXAMPLE OF BEACON COLLISION SCENARIO

In the case except the above-mentioned first example of the beaconcollision scenario, the beacon collision case is assumed. This isassumed to be a second example of a beacon collision scenario and isshown in FIG. 9. The second example is the example in which the systemsin which the networks were constructed already approach each other.

As shown in FIG. 9A, the communication station 10 (STA0) and thecommunication station 11 (STA1) exist in the range in which they cannotreceive radio waves from a communication station 12 (STA2) and acommunication station 13 (STA3, and the communication station 10 and thecommunication station 11 communicate with each other. Quiteindependently of the relationship between the above communicationstations, the communication stations 10 and 11 are communicating witheach other. Let it be assumed that the beacon transmission timings ofthe respective stations which are not aware of them at that timeunfortunately are overlapping with each other as shown on the right-handside of FIG. 9A. Also, assuming that the respective stations are movedlater and that they become able to transmit and receive information,then there occurs an accident in which the beacons of the respectivestations collide with each other as shown in FIG. 9B.

Such collision of the beacons can be avoided by the followingprocessing.

TBTT Offset Indicator (Offset Indicator):

FIG. 10 shows the TBTT times and the transmission times at which thebeacons are transmitted in actual practice.

The beacon transmission timing is determined at every 80 [msec] in thestep 1. The beacon transmission time determined at every 80 [msec] isdefined as a TBTT (Target Beacon Transmit Time). In this embodiment, inorder to prevent the beacons from colliding with each other continuouslyin the case like the above-mentioned second example of the beaconcollision scenario, the beacon transmission timing is displaced from theTBTT time intentionally. For example, when the TBTT offset is definedsuch that the actual beacon transmission time is set to any one of TBTT,TBTT+20 [μsec], TBTT+40 [μsec], TBTT+60 [μsec] TBTT+80 [μsec], TBTT+100[μsec], TBTT+120 [μsec] as shown in FIG. 10, the TBTT offset fortransmitting a beacon is determined at every superframe period and aTOISS field (which will be described later on) contained in the beaconis updated. Before a beacon is transmitted, the offset amount from theTBTT may be selected randomly this time.

While the beacon transmission time is defined at the unit of 20 [μsec]step, it is not limited to 20 [μsec] and may be defined by a smallerstep. The amount displaced from the TBTT intentionally is referred to asa “TBTT offset”.

Also, a TBTT Offset Indicator Sequence (TOISS) field shown in FIG. 15 isdefined as one of information transmitted by the beacon. In the TOISfield, there is written a beacon transmission offset value indicatingthat the amount in which the beacon is intentionally displaced from theTBTT this time and transmitted. The example of FIG. 11 shows the case inwhich there are provided seven stages of the TBTT offset values and theTOIS field is expressed as 3 bits “2 ̂3>=7”. When other packet istransmitted in the TBTT field, the beacon should be transmitted afterthe transmission of the above packet has been ended. It is frequentlyobserved that the beacon will not be transmitted at the time as thetransmission station intends to. In this case, a bit indicative ofTBTT+X is set as the TOIS field and the fact that this time beacontransmission timing is not the intended time is transmitted to theneighboring station which can receive the beacon.

As described above, since the beacon transmission time is displaced inaccordance with the TBTT offset, in the worst cases like “the secondexample of beacon collision scenario”, it is possible to avoid theaccident in which the beacon signals collide with each othercontinuously.

The TBTT offset can be given by the pseudorandom sequence such as a PNsequence. FIG. 12 is a diagram showing an example of a circuitarrangement in which the TBTT offset is generated by a 16-bitpseudorandom sequence (M sequence) that can be obtained by a simplecalculation. A bit string set to a register 80 is updated bit by bit tothe value obtained by the addition of adders 81, 82, 83, values areobtained from predetermined positions of the register 80 and added byadders 84 to 92, 3 bits are inputted to a register 93 and the 3 bits areset to the TBTT offset. According to this arrangement, it is possible toeffectively avoid the accident in which the beacon signals collide witheach other continuously.

While the definition of the TOIS field has been described so far as theinformation contained in the beacon, instead of the TOIS field, thecontent (TOI sequence) of the pseudorandom sequence register 80 shown inFIG. 12 will sometimes be transmitted as the information contained inthe beacon. When the content of the register 80 is transmitted as theinformation contained in the beacon, the reception station whichreceived that signal extracts information from the register 93 by themeans shown in FIG. 12 and can obtain the TOI information. The TOISfield is calculated each time the station transmits the beacon that isto be transmitted periodically. As a result, the station which receivedthe beacon once becomes able to calculate the TOIS information of thetransmission station in a free-running fashion to thereby obtain thenext offset and the next offset after the next TBTT offset before itreceives the beacon.

Also in this case, when the transmission station could not transmit thebeacon at the time as it intends to, the transmission station informsthe beacon reception station of the fact that the beacon transmissiontiming of this time is not the intended time by transmitting all zeroesas the TOI sequence (TOI Sequence).

Beacon Transmission Timing Alteration Request:

In the case of “the second example of beacon collision scenario”, therestill remains a problem in which beacons will collide with each otheronce in several times. Accordingly, when each station recognizes thatthe TBTT fields are set substantially simultaneously at a plurality ofstations, it can transmit a TBTT alteration request message to any oneof the beacon transmission stations. The communication station whichreceived such message scans the beacon of the neighboring station andsets a time at which the local station did not receive the beacon and atwhich 1 is not set by the NBOI field of the received beacon as a newTBTT (new TBTT). Before altering the TBTT field in actual practice afterthe new TBTT field was set, the communication station writes a messageof “new TBTT field is set and the TBTT field is altered after XX [msec]”in the beacon that is transmitted from the existing TBTT field andalters the TBTT field.

Countermeasure Against Difference of Clock Frequency:

Next, a mechanism for removing a difference of a clock frequencyoccurred between the respective communication stations will bedescribed. When the clock frequencies of the respective communicationstations are different, drift of transmission and reception timingsoccurs among the respective stations. If a difference up to ±20 ppm isallowed as an accuracy of a clock frequency, a clock frequency isdisplaced 3.2 [μsec] at 80 [msec]. If such displacement is left as itis, then there occurs an accident in which the beacon transmissiontimings overlap with each other. Accordingly, each communication stationcontinuously scans the beacons transmitted from the neighboring stationmore than once at about 4.0 [sec]. In that time period, it is desiredthat each communication station should receive over a time period longerthan the beacon transmission space of the local station. Then, thecommunication station matches the beacon transmission timing to the mostdelayed beacon transmission timing (TBTT) of the communication station.Although the clock frequency is displaced approximately 160 [μsec]during a time period of 4.0 [sec] at maximum, the communication stationcan make various countermeasures such as to control the timing withinthe local station after it has obtained displacement information.

In addition to the above-described object, the beacon scan is carriedout in order to confirm whether the state (presence) of a peripheraldevice is changed or not. Specifically, when the communication stationreceives a beacon from a new communication station during the beaconscan, the communication station transmits the message indicating thatthe new communication station appears to the high-order layer togetherwith information transmitted by the above beacon. Conversely, when thecommunication station could not receive the beacon from thecommunication station of which beacon could be received so far, thecommunication station stores therein such information. When thecommunication station could not receive the beacon from the samecommunication station over a plurality of scanning, it becomes awarethat the above communication station has left from the network and itinforms the high-order layer of such information. Alternatively, whenthe communication station could not receive the beacon from thecommunication station of which beacon could be received son far, itregards that the presence of the neighboring station was changed and itinforms the high-order layer of such information successively,whereafter it updates the list (Neighbor List) of the neighboringstation.

Next, details of an algorithm for countermeasure against difference ofclock frequencies will be described with reference to a flowchart ofFIG. 13. Clock frequency difference information is obtained by beaconscanning. When beacon scanning (countermeasure processing againstdifference of clock frequencies) is started, first, a timer is set tostart counting 80 [msec] which is the beacon space. Then, it isdetermined whether or not this count is ended (step S1). When the countis ended, the beacon scanning and information collection required forclock frequency difference countermeasure is ended. The communicationstation continues to attempt to receive the beacon until the timer isended. If the beacon is received (step S2), then the communicationstation compares the TBTT field calculated within the local station andthe TBTT field of the received beacon with each other. The communicationstation can obtain the TBTT field of the received beacon by examiningthe time at which the beacon is received and the TOIS field. When theTOIS field is set as TBTT+X, that beacon received time is omitted fromthe total target.

When the TIOS sequence is written in the beacon, all bits are set to 0as the notation indicating TBTT+X. If the station which received thishas the TIOS sequence in which all bits are 0, such beacon received timeis omitted from the total target.

The communication station calculates “delayed amount of the TBTT fieldof the received beacon from the TBTT field calculated within the localstation” with respect to the beacon of the total target (step S3). Then,the beacon of which TBTT field is most delayed is judged from all thebeacons received until the timer is ended (step S4), and this delayedamount is stored as a most delayed timing (Most Delayed Timing: MDT)(step S5). A value which results from subtracting a previously-set a[μsec] (for example, 2 [μsec]) from the MDT obtained at the time atwhich the timer is ended is set to α (step 6). Then, it is determinedwhether or not α is a positive number, that is, the value which resultsfrom subtracting a [sec] from the MDT is delayed from the clockfrequency of the local station (step S7). If delayed, then the clockfrequency of the local station is delayed α (step S8).

According to the above processing, even when the clock frequency of eachcommunication station is displaced, a time is fundamentally adjusted inaccordance with the most delayed clock frequency of the communicationstation existing within the system and hence it is possible to avoid theaccident in which transmission and reception timings will drift andoverlap with each other. The above-described value a [μsec] is the valuethat should be set in accordance with the specification required fortiming control and may not be limited herein.

The scan space is first set to be a relatively short space of about 1[sec]. When the above-described clock drift value is extracted, if it isdetermined that disagreement between the clock frequency of the localstation and the clock frequency of the neighboring station is not soremarkable, then it is possible to further suppress the influence causedby the clock drift by using a method for setting a longer spacestepwise.

Stop Receiving Beacon of Specific Station:

Although each communication station receives the beacon transmitted fromthe neighboring station in accordance with the above-describedprocedure, when it receives from the high-order layer an instructionmessage of “stop communication with this communication”, it does notperform reception operation at the beacon transmission time of thecommunication station. As a result, it becomes possible to decreaseunnecessary reception processing between it and a communication stationwhich is not relating to the local station. Hence, it becomes possibleto contribute to decrease of power consumption. The instruction messageof “stop communication with this communication station” is judged fromattribute of devices of the communication station, it is issued whenauthentication was not made or it is instructed by users.

Definition of Packet Space (Inter Frame Space):

Similarly to the cases such as the IEEE802.11 system, a plurality ofpacket spaces is defined also in this example. The definition of thispacket space will be described with reference to FIG. 14.

As the packet space, there are defined an SIFS (Short Inter Frame Space)which is a short packet space and an LIFS (Long Inter Frame Space) whichis a long packet space. Only a prioritized packet is allowed to betransmitted at the SIFS packet space, and other packets are allowed tobe transmitted during the random backoff packet space in which the LIFS+random value is obtained after it has been determined that the media isclear. To calculate the random-backoff value, there is used a methodthat is known in the existing technology.

Further, in this embodiment, “LIFS” and “FIFS+backoff” (FIFS: Far InterFrame Space) are defined in addition to the above-mentioned packetspaces “SIFS” and “LIFS+backoff”. Although it is customary to apply thepacket spaces of “SIFS” and “LIFS+backoff”, in a time zone in which acertain communication station is given a prioritized transmission right,other communication station uses the packet space of “FIFS+backoff” andthe communication station which is given the priority uses the packetspace of SIFS or LIFS. The paragraph “time zone in which a certaincommunication station is given a prioritized transmission right” will bedescribed below.

Transmission Prioritized Interval TPP:

While each communication station is transmitting a beacon at a constantspace, according to this embodiment, during a proper time period afterthe transmission of the beacon, the communication station that hastransmitted the beacon is given a prioritized transmission right. FIG.15 shows an example of the manner in which the beacon transmissionstation is given a prioritized transmission right. FIG. 16 shows anexample in which 480 [μsec] is given as this transmission prioritizedinterval. This prioritized interval is defined as TPP (TransmissionPrioritized Period). The TPP is started immediately after the beacon wastransmitted and ended at a time passed from the TBB field by T_TGP.Since each communication station transmits the beacon at everysuperframe, the TPP with the same time rate is fundamentally distributedto each communication station. A time period in which othercommunication station transmits the beacon after the TPP of onecommunication station elapsed is served as an FAP (Fairly AccessPeriod).

In the FAP (Fairly Access Period), there is carried out a fair mediaacquisition contention based upon the ordinary CSMA/CA system (orPSMA/CA system which will be described later on).

FIG. 16 shows an arrangement of a superframe. As illustrated, after eachcommunication station has transmitted the beacon, the TPP of thecommunication station which has transmitted such beacon is assigned, theFAP is assigned after a time corresponding to the duration of the TPPelapsed and the FAP is ended when the next communication stationtransmits the beacon.

While the TPP is started immediately after the beacon was transmitted byway of example, the present invention is not limited thereto and thestart time of the TPP may be set to a relative position (time) from thebeacon transmission time. Also, the TPP may be defined in the form of480 [μsec] from the TBTT field. Further, as shown in FIG. 15, since theTGP area is expired during the period T_TPP that is based on the TBTTfield, when the beacon transmission time is delayed due to the TBTToffset, the TPP area is reduced.

Here, a packet space in each field within the superframe will bedescribed. During the FAP period, all communication stations cantransmit the beacons at the “LIFS+backoff” space and hence the accessright can be acquired by fair contention control. For example, in orderto acquire the access right, the RTS packet and short commands aretransmitted at the “LIFS+backoff” space and the CTS packet, data and theAck packet which are to be transmitted later are transmitted at the“SIFS” space. IFS parameters in the FAP will be shown below.

TABLE SETTING OF IFS PARAMETER IN FAP KIND OF COMMUNICATION ACCESS WAITKIND OF TRANSMISSION STATION STATE FRAME SPACE TRIGGER ALL HAVING RTSLIFS + Backoff N/A COMMUNICATION TRANSMISSION COMMAND LIFS + Backoff N/ASTATIONS DATA EXISTS N/A CTS SIFS SPACE AFTER RTS WAS RECEIVED HAVINGDATA SIFS SPACE AFTER CTS WAS TRANSMISSION RECEIVED DATA EXISTS DATARECEIVED ACK SIFS SPACE AFTER DATA WAS RECEIVED

On the other hand, in the TPP area, the communication station whichtransmitted the beacon is given the access right and is allowed totransmit the frame after SIFS time passed. Also, the communicationstation which is designated by the communication station whichtransmitted the beacon is given a prioritized transmission right and isallowed to transmit the frame after the SIFS time elapsed. When ananswer to the CTS packet is not received although the communicationstation which acquired the prioritized transmission right transmits theRTS packet to a specific communication station, the communicationstation which acquired the prioritized transmission right transmitsagain the RTS packet at the LIFS space.

Also, when another communication station that holds data to betransmitted to the communication station which acquired the prioritizedtransmission right confirms the message “node does not have transmissiondata”, it allows the transmission at the SIFS+backoff (Backoff) framespace. However, it is frequently observed that the third communicationstation has no means to recognize that the communication station whichacquired the prioritized transmission right has data.

The communication station without the prioritized transmission rightrecognizes by receiving the beacon that other communication stationstarts the prioritized transmission, it sets the fundamental frame spaceto the FIFS during the period of T_TPP and it tries to acquire theaccess right at the FIFS+backoff frame space.

By the above-described procedure, there is realized a mechanism in whichwhen the communication station which acquired the prioritizedtransmission right in the TPP area by the above-described procedure hasthe data which the communication station is to transmit and receive,that communication station is given the access right while when theabove communication station does not have the data to be transmitted andreceived, the access right of the communication station is discarded andother communication station acquires the access right.

The following controls are required depending upon the kinds and statesof the respective communication stations.

TABLE SETTING OF IFS PARAMETER IN TPP KIND OF COMMUNICATION ACCESS WAITKIND OF TRANSMISSION STATION STATE FRAME SPACE TRIGGER WITH ACCESS SETPRIORITY RTS SIFS SPACE AFTER BEACON RIGHT TRANSMISSION COMMAND WASRIGHT TRANSMITTED WITH PRIORITY N/A CTS SIFS SPACE AFTER RTS WASTRANSMISSION COMMAND RECEIVED RIGHT WITHOUT PRIORITY TRANSMIT BEACON RTSSIFS + Backoff AFTER COMPLETION TRANSMISSION TRANSMISSION DATA COMMANDOF TRANSMISSION RIGHT TO COMMUNICATION OF PRIORITY STATION EXISTSTRANSMISSION RIGHT COMMUNICATION STATION TRANSMIT BEACON RTS FIFS +backoff AFTER COMPLETION TRANSMISSION DATA COMMAND OF TRANSMISSION OF TOCOMMUNICATION PRIORITY TRANSMISSION STATION DOES NOT RIGHT COMMUNICATIONEXIST STATION ALL TRANSMISSION DATA SIFS SPACE AFTER CTS WASCOMMUNICATION DATA EXISTS RECEIVED STATIONS DATA RECEIVED ACK SIFS SPACEAFTER DATA WAS RECEIVED

With respect to the packet transmission within the TPP of the localstation, the communication station is also allowed to transmit thepacket at the LIFS space. Further, with respect to the packettransmission within the TPP of other station, other station transmitsthe packet at the FIFS+backoff space. While the FIFS+backoff isconstantly used as the packet space in the IEEE802.11 system, accordingto the arrangement of this embodiment, this space can be narrowed andhence more effective packet transmission becomes possible.

Also, while each communication station fundamentally transmits onebeacon at every superframe period, depending on the cases, it is allowedto transit a plurality of beacons or a signal similar to the beacon, andit can acquire the TPP each time it transmits these beacons. In otherwords, the communication station can maintain the prioritizedtransmission resources corresponding to the number of the beaconstransmitted at every superframe. Here, the beacon that the communicationstation constantly transmits at the beginning of the superframe periodis referred to as a “regular beacon” and the beacon following the secondbeacon that is transmitted at other timing in order to acquire the TPPor for other purposes is referred to as an “auxiliary beacon”.

Application of Use of TPP:

When the TPP is defined as 480 [μsec], 21 packets corresponding to 60[Byte] or about one packet of 6000 [Byte] can be transmitted.Specifically, even when the media is crowded, transmission ofapproximately 21 ACK packets at 80 [msec] can be guaranteed.Alternatively, when only the TPP is used, a transmission line of 600[kbps]=(6000 [Byte]/80 [msec]) can be maintained at the lowest. Whilethe prioritized transmission right is given to the communication stationin the TPP as described above, the prioritized transmission right isgiven to a communication station which is called by the communicationstation in the TPP. While transmission takes precedence in the TPPfundamentally, when the communication station does not have anyinformation to be transmitted but it is clear that other station hasinformation that is to be transmitted to the local station, a paging(Paging) message or a polling (Polling) message may be transmitted tosuch “other station”.

Conversely, when the local station has no information to be transmittedalthough it has transmitted the beacon and the communication station isnot aware of the fact that other station has information to betransmitted to the local station, the above communication station doesnothing, it discards the transmission priority given thereto by the TPPand it does not transmit any information. Then, after the LIFS+backoffor the FIFS+backoff passed, other station starts transmission even inthis time zone.

Having considered the arrangement in which the TPP is followedimmediately after the beacon was transmitted as shown in FIG. 16, it ispreferable that the beacon transmission timing of each communicationstation should be equally dispersed within the transmission frame periodrather than crowded. Accordingly, in this embodiment, fundamentally, thetransmission of the beacon is started at substantially the center of thetime zone in which the beacon space is longest within the range in whichit can receive the beacon. Of course, there may be used a method inwhich the beacon transmission timing of each communication station islocated intensively and reception operation is stopped during theremaining transmission frame period to decrease power consumption.

Field of Beacon:

Information described in the beacon transmitted in the decentralizeddistributed type wireless communication system according to thisembodiment will be described. FIG. 18 shows an example of a beaconsignal format.

As was already been described with reference to FIG. 17, the preambleindicative of the existence of the packet is added to the beginning ofthe packet, the heading area in which attribute and length of the packetare described exists next to the preamble and the PSDU is coupled to theheading area. When the beacon is transmitted, information indicating amessage in which the packet is the beacon is written in the headingarea. Also, information that is transmitted by the beacon is written inthe PSDU.

In the illustrated example, the beacon contains a TA (TransmitterAddress) field which is an address indicating the transmission stationuniquely, a Type field indicative of the kind of the beacon, a TOI fieldindicative of a TBTT offset value in the superframe period during whichthe beacon is transmitted, an NBOI (Neighboring Beacon OffsetInformation) field which is reception time information that can bereceived from the neighboring station, an NBAI (Neighboring BeaconActivity Information) field which is information indicative of atransmitted time of a beacon signal received by the local station, anALERT field which stores therein information for altering the TBTT fieldor other various kinds of information to be transmitted, a TxNum fieldindicative of an amount in which the communication station maintainsresources with a priority, a Serial field indicative of an exclusiveunique serial number assigned to the beacon when a plurality of beaconsis transmitted during the superframe period, a TIM (Traffic IndicationMap) field which is information indicating that the destination stationto which information of this communication station is transmitted atpresent, a Page (Paging) field indicating that the reception stationwritten in the TIM field plans to transmit information in theimmediately-following TPP, a Sense Level field for storing thereininformation indicating the level (reception SINR) of the receptionsignal that the station detects as the reception signal, a TSF (TimingSynchronization Function) field for reporting time information includedin the station and a NetID (Network Identifier) field that is anidentifier such as an owner of the station and so on.

The kind of the beacon is described in the Type field in the bit mapformat of 8-bit length. In this embodiment, information which determineswhether the beacon is “regular beacon” that each communication stationtransmits once at the beginning of the superframe at every superframe or“auxiliary beacon” that is transmitted to acquire the prioritizedtransmission right is shown by using the values ranging from 0 to 255which show a priority. To be concrete, 255 which show the maximumpriority is assigned to the regular beacon that should be transmittedonce at every superframe, and any one of 0 to 255 that corresponds tothe priority of the traffic is assigned to the auxiliary beacon.

The pseudorandom sequence that determines the above-mentioned TBTToffset is stored in the TOI field and it indicates the amount of theTBTT offset with which the beacon is transmitted. Since the TBTT offsetis provided, even when two communication stations locate the beacontransmission timing at the same slot on the superframe, the actualbeacon transmission timing can be displaced. Thus, even when the beaconscollide with each other in a certain superframe period, the respectivecommunication stations can listen to thief beacons (or neighborcommunication stations can listen to their beacons) in anothersuperframe period, that is, they can recognize the collision of thebeacons.

The NBOI field is the information in which the position (reception time)of the beacon of the neighboring station that the local station canreceive in the superframe is described. In this embodiment, since onesuperframe has the slots into which 16 beacons can be located at maximumas shown in FIG. 4, information concerning the layout of the beaconsthat could be received is described in the bit map format of 16-bitlength. That is, the leading bit (MSB) of the NBOI field is mapped withreference to the transmission time of the regular beacon of the localstation, the position (reception time) of the beacon that can bereceived by the local station is mapped on the bit of the relativeposition from the transmission time of the regular beacon of the localstation, 1 is written in the bit corresponding to the relative position(offset) of the regular or auxiliary beacon of the local station and therelative position (offset) of the beacon that can be received and thebit position corresponding to other relative position remains to be 0.

For example, under the communication environment in which 16communication stations 0 to F at maximum are accommodated as shown inFIG. 4, when the communication station 0 makes the NBOI field such as“1100, 0000, 0100, 0000”, this communication station can transmit amessage “it is able to receive the beacons from the communicationstations 1 to 9”. That is, “1” is assigned to the bit corresponding tothe relative position of the beacon that can be received when the beaconcan be received and “0”, that is, space is assigned thereto when thebeacon is not received. Also, the reason that the MSB is “1” is that thelocal station transmits the beacon and hence “1” is assigned to theportion corresponding to the time at which the local station transmitsthe beacon.

The position (reception time) of the beacon that the local stationreceives in actual practice is described in the NBAI field at itsrelative position from the beacon position of the local station by thebit map format. That is, the NBAI field indicates that the local stationis set to the active state in which it can receive information

Information to be transmitted to the neighboring station is stored inthe ALERT field in the abnormal state. For example, when it is plannedto change the TBTT field of the regular beacon of the local station inorder to avoid the collision of the beacons or when it is requested tostop the neighboring station from transmitting the auxiliary beacon,such message is described in the ALERT field. The manner in which theALERT field is in actual use will be described later on.

The number of the auxiliary beacons that the station is transmittingwithin the superframe period is described in the TxNum field. Since thecommunication station is given the TPP, that is, the prioritizedtransmission right after the transmission of the beacon, the number ofthe auxiliary beacons within the superframe period corresponds to a timerate in which the communication station maintains the resources with apriority to transmit information.

A serial number assigned to the beacon when a plurality of beacons istransmitted within the superframe is written in the Serial field. As theserial number of the beacon, an exclusive and unique number is assignedto each beacon that is transmitted within the superframe. In thisembodiment, a serial number indicative of the sequential order of theTBTT field in which the auxiliary beacon is transmitted based on theregular beacon of the local station is written in the Serial field.

Report information indicating the destination station to which thiscommunication station has information to be transmitted at present isstored in the TIM field. It is possible for the reception station torecognize that the local station should receive information withreference to the TIM field.

Also, the Paging field is the field indicative of the reception stationdescribed in the TIM field to which the communication station intends totransmit information in the immediately-succeeding TPP. The stationdesignated by this field should become ready to receive information inthe TPP field, and other field (ETC field) is also prepared.

A TSF field is a field in which time information included in the stationis transmitted. This time is used for other uses than the media accessand is mainly used to synchronize the applications. The transmissiontime of the signal that is calculated faithfully, in a free-runningfashion, to the clock frequency of the transmission stationindependently of the access control such as the alteration of thetransmission time of the beacon, the correction of the clock frequencyto hold the TDMA structure and the TBTT offset is written in this field.The reception station supplies this value to the high-order layertogether with the reception time and may hold it as the reference timeof the information transmitted from the station.

The NetID field is an identifier indicating the owner of thecorresponding station. The reception station can recognize withreference to this field whether or not the local station and thecorresponding station belong to the same network logically.

Procedure of Transmitter and Receire in the Stationary State No. 1:

A typical example of transmission and reception procedures of acommunication station will be described with reference to FIG. 19. FIG.19 shows a communication station STA0 and a communication station STA1in the case in which the communication station STA0 transmitsinformation to the communication station STA1. Each communicationstation does not always receive a beacon signal from other station everytime. A frequency at which the communication station receives the beaconsignal may be lowered by an instruction from the high-order layer andthe like. FIG. 19A shows a sequence diagram of a packet transmitted andreceived between the communication stations STA0 and STA1, FIG. 19Bshows the state of the transmission unit of the communication stationSTA0, and FIG. 19C shows the state of the reception unit of thecommunication station STA0. In the state of the transmission andreception unit, the high level state indicates the active state (statein which the transmission and reception unit attempts to receive ortransmit information) and the low level state indicates the sleep state.

First, having confirmed that the media is clear, the communicationstation STA0 transmits a beacon. Let it be assumed that thecommunication station STA1 is called in the TIM field and (or) PAGEfield in this beacon. The communication station STA1 which received thebeacon generates a response to paging information (0). Since thisresponse corresponds to the TPP of the communication station STA0, it isgiven a priority and it is transmitted at the SIFS space. After that,transmission and reception between the communication stations STA1 andSTA0 within the TPP is given a priority and hence this response istransmitted at the SIFS space. The communication station which receivedthe response transmits a packet to the communication station STA1 afterit has confirmed that the communication station STA1 is placed in thereceivable state (1) Further, in FIG. 19, there exists another packet tothe communication station STA1, and hence another packet is transmitted(2). The communication station STA1 which received the two packetstransmits the ACK packet after it has confirmed that the two packetswere received correctly (3). Thereafter, the communication station STA0transmits the last packet (4). However, during the communication stationis receiving the ACK packet, the TPP field of the communication stationSTA0 is ended and the communication station enters the FAP field when ittransmits the last packet (4). Since the communication station does nothave the prioritized transmission right in the FAP field and thecommunication station transmits the last packet (4) at the LIFS+backoffspace. The communication station STA1 transmits the ACK packetcorresponding to the last packet (4) (5).

A time period from the last transmission is defined as a “listen period”(Listen Period) in which each communication station is ought to energizethe receiver. FIG. 19 also shows this state. When the reception packetdoes not exist during the listen period, the communication station ischanged to the sleep mode and it de-energizes the transmitter andreceiver to decrease power consumption. However, when the communicationstation receives in advance some message indicating “DO NOT WISH TOCHANGE TO SLEEP MODE” from other station or when the communicationstation receives a similar message from the high-order layer, thecommunication station is not limited to the above operation butcontinues to operate the reception unit.

The communication station, which was once placed in the sleep mode,releases the sleep mode in response to a time at which information istransmitted and received next time such as when the communicationstation receives a beacon from other station or it transmits the beaconof the local station and returned to the active state. In the example ofFIG. 19, although the communication station is temporarily returned tothe active mode in order to receive the beacon from the communicationstation STA1, after it was confirmed that the packet to be transmittedto the communication station STA1 does not exist in the TIM field andthe PAGE field of the beacon transmitted from the communication stationSTA1, the communication station is again placed in the sleep mode. Afterthat, before transmitting the beacon of the local station, thecommunication station energizes the reception unit for sensing the mediaand after it was confirmed that the media is clear, it transmits thebeacon. Although the communication station does not access othercommunication station in the TIM field and the PAGE field when ittransmits the beacon this time, since the communication station STA0transmits the beacon, the communication station enters the listen periodin accordance with the above-described procedure after it hastransmitted the beacon and monitors for a while whether or not a signalto the local station is received. When the communication station doesnot receive any signal and the listen period is ended, it changes itsmode to the sleep mode again.

Summary of Example of Transmission and Reception No. 1:

When the communication station transmits the signal, the transmission ofthe signal is started by the access of the beacon. Having transmittedand received the last packet, the communication station attempts toreceive a signal for a while. When the packet does not arrive at thelocal station, the communication station enters the sleep mode (sleepstate). Each time the communication station receives the beacon fromother station or transmits the beacon of the local station, it isreturned to the active mode (active state). That is, during a stipulatedtime period after the communication station has transmitted some signal,it energizes the reception unit (communication unit) constantly.

Procedure of Transmitter and Receiver in the Stationary State No. 2(Paging Transfer Sequence):

Another typical example of the transmission and reception procedures ofthe communication station will be described with reference to FIG. 20.Each communication station does not always receive a beacon every time.It is frequently observed that a reception frequency may be lowered byan instruction from the high-order layer and the like. The transmissionand reception procedures in this case will be described. FIG. 20 showsthe communication stations STA0 and STA1 in which the communicationstation STA1 transmits a signal to the communication station STA0, byway of example. FIG. 20A shows a sequence diagram of a packettransmitted and received between the communication stations STA0 andSTA1, FIG. 20B shows the state of the transmission unit of thecommunication station STA0, and FIG. 20C shows the state of thereception unit of the communication station STA0. In the state of thetransmission and reception unit, the high level state indicates theactive state (state in which the communication station is attempting toreceive or transmit a signal) and the low level state indicates thesleep mode.

Having confirmed that the media is clear, the communication station STA1transmits a beacon. At that time, the communication station STA0 isplaced in the sleep mode and is not in receipt of the beacon.Accordingly, even when the communication station STA0 is accessed in theTIM field and (or) PAGE field, the communication station STA0 does notrespond to such accessing. Thereafter, the communication station STA0transmits the beacon at the beacon transmission time of the localstation. Each time the communication station STA1 receives the beaconfrom the communication station STA0, it transmits paging information tothe communication station STA0 in accordance with the determined randombackoff procedure. Having transmitted the beacon, the communicationstation STA0 is energizing the receiver during the listen period andhence it can receive this paging information. That is, when receivingthe paging information, the communication station STA0 can recognizethat the communication station STA1 has information for the localstation.

At that time point, the communication station STA0 may make a responseto the paging information of the communication station STA1 and thecommunication station STA0 may start transmitting information to thecommunication station STA1 (although not shown). FIG. 20 shows anexample of the case in which the communication station does not yetstart transmitting information at that time. After that, at the beacontransmission time of the communication station STA1, the communicationstation STA0 is caused by the previous paging information to attempt toreceive information from the communication station STA1 and it receivesthe beacon from the communication station STA1. Let it be assumed thatthe communication station STA0 is accessed in the TIM field and (or)PAGE field in the beacon. Then, the communication station STA0 whichreceived this beacon makes a response to the paging information (0).This response corresponds to the TPP of the communication station STA1and the communication station is given the prioritized transmissionright and it transmits information at the SIFS space. After that, thetransmission and reception between the communication stations STA1 andSTA0 within the TPP field is given the prioritized transmission rightand hence information is transmitted at the SIFS space. When thecommunication station STA1 which received the response recognizes thatthe communication station STA0 is placed in the receivable state, ittransmits the packet to the communication station STA0 (1). Thecommunication station STA0 which received this packet recognizes thatthe packet was received correctly and transmits the ACK packet (2).Thereafter, the communication station STA0 energizes the receiver duringthe listen period to confirm that the packet to the local station is notreceived, and it changes its state to the sleep mode.

While the packet is transmitted to the beacon transmission station eachtime the communication station starts receiving the beacon on theassumption that the receiver is being operated during the listen periodas described above, the present invention is not limited thereto. Whenmedia sense is performed before the beacon transmission time, it isclear that the receiver is being operated before the beacon transmissiontime. Thus, even when transmission processing is executed at this timezone, similar effects can be achieved.

Summary of Example of the above Transmission and Reception Procedure No.2:

When a signal is transmitted, paging information is transmittedimmediately after the beacon has been transmitted from the receptionside, whereby the reception side is changed into the active state tostart transmission and reception processing. Alternatively, thetransmission and reception processing is started in response to theaccess by the beacon from the transmission side. Then, after the lastpacket was transmitted and received, the reception unit attempts toreceive information for a while. If the packet to the local station doesnot arrive at the communication station, the communication station isplaced in the sleep move and each time it receives the beacon from otherstation or it transmits the beacon of the local station, thecommunication station is returned to the active mode. That is, thecommunication station transmits paging information during the listenperiod of the reception side or in the media sense interval prior to thetransmission of the beacon.

Although the message transmitted immediately before/immediately afterthe reception side transmits the beacon in the above-described receptionprocedure 2 is not limited to the paging information, since there is apossibility that contention of access of messages from a plurality ofstations will occur, it is desired that only a message with a largeemergency such as paging information and a beacon transmission timingalteration request should be transmitted.

While the present invention has been described in the form in which theRTS/CTS procedure executed prior to the transmission of the packet forsimplicity of description as described above, according to thenecessity, the RTS packet and the CTS packet may be exchanged before thepacket is transmitted. In that case, it is needless to say that thepaging information in the beacon corresponds to the RTS packet and thepage response corresponds to the CTS packet.

Also, while the paging information and negotiation processing of itsresponse are executed between the communication stations beforetransmission of data is started in the above-mentioned example, thepresent invention is not limited thereto and the source communicationstation which holds data to be transmitted to a certain communicationstation may start transmitting data without negotiation processingwithin the listen period of the reception communication station or anactive timing in which such communication station is performing thereception operation (Active Transfer sequence). In this case, processingfor establishing connection can be omitted and communication becomeshighly efficient.

Application of Process for Determining Beacon Transmission Timing:

The beacon transmission timing will be described. First, the beacontransmission timing will be described with reference to FIGS. 21 and 22.

For example, let it be assumed that two communication stations ofcommunication stations STA-0 and STA-1 exist within the beacon radiowave reaching range. In this case, beacons B0, B1 are locatedsubstantially alternately and they are located at a timing relationshipof approximately 40 [msec] space as shown in FIG. 21. When an amount oftransmission data of the communication stations STA-0 and STA-1 is notso large, the communication station STA-0 starts transmitting thetransmission signal in response to the start of the transmission of thebeacon from the communication station STA-0 and the transmission isended after a while. The transmission signal from the communicationstation STA-1 is similar and if the transmission information amount isended in a time period shorter than the space of beacons, then it isexpected that the transmission requests from the communication stationsSTA-0 and STA-1 will not collide with each other.

FIG. 22 shows the case in which three communication stations existwithin the beacon radio wave reaching range similarly.

Here, we assume the case in which a new communication station STA-2joins this beacon radio wave reaching range. The beacon transmissiontiming of the communication station STA-2 may be either 20 [msec] or 60[msec] shown in the sheet of drawing. However, the communication stationSTA-2 scans the media state before it determines the beacon transmissiontiming. When traffics are packet transmission P0 that follows the beaconB0 and packet transmission P1 that follows the beacon B1 shown in FIG.22, if the communication station STA-2 transmits a beacon B2 at a timingof 20 [msec], then collisions of the beacons will be decreased. Fromthis standpoint, it becomes possible for the communication station STA-2to determine the beacon transmission time in consideration of theoccupied state of the media, that is, the traffic amount of eachcommunication state. This is especially effective for the case in whichtransmission activity becomes different considerably depending upon thecommunication station.

Band Reservation for Transmitting Stream Data:

Further, let us consider the case in which a communication station whichtransmits stream data of wide band exists within the system. Thecommunication station intends to continuously transmit a signal of aconstant band without collision. In this case, the transmission stationincreases a beacon transmission frequency within the superframe period.An example of this case will be described with reference to FIG. 23.

It is customary that the superframe period in the channel is defined bythe beacon space. In this embodiment, the beacons following the secondbeacon in one superframe period is transmitted mainly in order to obtaintransmission and reception intervals and hence they are different fromthe original beacons that are transmitted to configure the network froma nature standpoint. In this specification, the beacons following thesecond beacon in one superframe period are referred to as “auxiliarybeacons”.

On the other hand, a minimum beacon space Bmin is stipulated in order toprevent the band (superframe period) from being filled with beacons andthere is an upper limit on the number of communication stations that canbe accommodated within the superframer period (mentioned hereinbefore).For this reason, when a new communication station joins the network, theauxiliary beacon has to be released in order to accommodate this newcommunication station in the superframe period.

While FIG. 23 shows the case in which the beacons B1 and B1′ aretransmitted continuously, the present invention is not limited thereto.When the communication station transmits the beacon, the beacon isimmediately followed by the TPP field and it becomes possible to acquirethe media without access acquisition contention. A communication stationwhich strongly requires the right of possessing the media can get muchmore transmission rights by increasing the frequency at which the beaconis transmitted.

Also, the “auxiliary beacon” need not always describe thereon beaconinformation. In order to decrease the overhead in which the beacons aretransmitted a plurality of times, a packet category called a “falsebeacon for accommodating traffic” may be defined in which a flag of amessage indicating that attribute of a packet is a kind of a beacon maybe raised and traffic may be transmitted as the contents.

For example, in a certain system, when the capacity reachessubstantially its limit and quality of services that the network isproviding at present cannot be guaranteed if much more traffic isaccommodated, each communication station transmits as much beacons aspossible. Thus, even when a new communication station joins the network,the beacon transmission timing cannot be given to such new communicationstation and the accommodation of the new communication station into thisarea can be refused.

Example of Use of Quiet (Quiet) Packet:

While each station transmits a beacon periodically, since the trafficpacket is transmitted in accordance with the CSMA (or PSMA) procedure,an accident will be caused by the transmission of the traffic packetfrom other station in which the beacon cannot be received. FIG. 24 showsan example of this case.

In FIG. 24, it is assumed that, when communication stations STA1, STA2,STA3, STA4 exist, the communication station STA2 transmits informationto the communication station STA1, the communication station STA3 existsin the area in which the transmission signal from the communicationstation STA2 can be received, the communication station STA3 intends toreceive the beacon transmitted from the communication station STA4 andthat the communication station STA2 exists in the area in which itcannot receive the beacon from the communication station STA4. In thisexample, at a time T0, the communication station STA4 transmits thebeacon and the communication station STA3 starts receiving thistransmitted beacon. However, since the communication station STA2 cannotreceive the signal from the communication station STA4, thiscommunication station starts transmitting information to thecommunication station STA1 at a time T1 in accordance with the randombackoff procedure. The transmitted signal from this communicationstation STA2 interferes with the communication station STA3 so that thiscommunication station becomes unable to receive the beacon from thecommunication station STA4.

A quiet (Quiet) packet is used in order to avoid this accident. Thequiet packet is a packet which transmits a message “This station willreceive information from other station and wishes other station not totransmit a signal” to the neighboring stations. As shown in FIG. 25, thequiet packet describes thereon “target station of which information willbe received by quiet packet transmission station (target)” and“transmission prohibit time”.

In the example of FIG. 24, the communication station STA3 transmits thequiet packet at the time T3 before a time T4 which is the next TBTTfield of the communication station STA4. When the communication stationSTA2 which received the quiet packet recognizes that the local stationis not the target station of the quiet packet, it stops transmittinginformation until the time instructed by the quiet packet. On the otherhand, although the quiet packet reaches the communication station STA4,when the communication station STA4 recognizes that the local station isthe target station of the quiet packet, it neglect the quiet packet andtransmits the beacon at the time T4 that is the TBTT field as it isplanned to do. Thus, the communication station STA3 becomes able toreceive the beacon without being disturbed by the communication stationSTA2.

Example of Operation of Media Scan Method (PSMA: Preamble Sense MultipleAccess):

This embodiment uses the CSMA procedure as the access method and hencethe fundamental method is to transmit information after it has confirmedthe communication state. However, in the specification of the physicallayer of the baseband unit of the communication station, such a case isconsidered in which information such as a reception electric fieldintensity (RSSI) cannot be used as media occupied information. Forexample, this case may be a communication system such as an ultra-wideband communication for making communication by using a wide band rangingfrom 30 GHz to 10 GHz. In such case, the existence of the packet can berecognized only by receiving the preamble of the unique word added tothe leading portion of the packet. That is, this media scan method iscollision avoidance control based upon the detection of the preamble andthe transmission station transmits information after it has confirmedthat the media state is clear. This is defined as “PSMA”. For thisreason, even when the transmission station which intends to transmitinformation after it has changed from the sleep mode transmits anyinformation, it starts media reception processing before a time of apredetermined time (MDI: Maximum Data Interval: maximum data interval(that is, maximum packet length)). When the above communication stationdetects the preamble of the packet transmitted from other communicationstation during this time period, it refrains from transmittinginformation.

Since the communication station performs access control by detecting thepreamble, the preamble is constantly added to the PHY frame. FIG. 26shows a PHY frame format stipulated by a PHY layer (physical layer). Thepreamble added to the beginning of the PHY frame is composed of theknown unique word.

The communication station which receives information and thecommunication station which transmits information can recognize bydetecting the preamble that the media is occupied. This state will bedescribed with reference to FIG. 27. FIG. 27 is a diagram used toexplain the case in which the communication stations STA0 and STA1transmit information. FIG. 27A shows a transmission sequence of thecommunication station STA1, and FIG. 27B shows a transmission sequenceof the communication station STA0.

Then, FIGS. 27C, 27D show the states of the transmission unit and thereception unit of the communication station STA0 (high level: activemode, low level: sleep mode).

At a time T1, the communication station STA1 starts transmitting apacket. Since the communication station STA0 is in the sleep mode atthat time point, it is unable to recognize that the communicationstation STA1 has transmitted the packet. Thereafter, let it be assumedthat the high-order layer informs of the communication station that thecommunication station STA0 has information to be transmitted at the timeT1 (Tx request). Although the random backoff procedure is started atthis time point according to the conventional IEEE802.11 system wirelessLAN, since the communication station starts receiving information fromthe time T1, it cannot receive the preamble of the unique word and henceit cannot recognize that the media is being used by the communicationstation STA1. Therefore, there is then a possibility that thetransmission of information from the communication station STA0 willinterfere with the packet of the communication station STA1.Accordingly, when the communication station STA0 is placed in the activemode at the time T1, from this time point, it confirms during themaximum data space MDI (Max. Uniqueword Interval) that the media isclear. A time T2 is a time point that passed from the time T1 by MDI.The communication station STA0 energizes the receiver from the time T1to the time T2 and starts transmitting information only when it does notdetect the unique word (preamble of FIG. 25) of the packet.

Let it be assumed that the high-order layer reports information to thecommunication station (Tx request). Since the communication station STA0is set to the sleep mode immediately before the time T4, thecommunication station starts confirming during the time period from thetime T4 to the MDI that the media is clear. Then, since the packet istransmitted from the communication station STA1 at a time T5 this time,the communication station STA0 detects the unique word to recognize theexistence of this packet. The communication station STA0 starts therandom backoff procedure from a time T6 at which the transmission ofthis packet is ended. If the communication station does not detect theunique word until a time T7 at which the timer is de-energized, then ittransmits the packet at the time T7.

While the present invention has been described so far on the assumptionthat the MDI is equal to the maximum packet length, when thecommunication station intends to transmit a large amount of data thatcannot be transmitted by one packet, data transfer over a long period oftime may be allowed by acquiring the access right once as shown in FIG.28. As shown in FIG. 28, within the range of the maximum datatransmission length obtained when the access right is acquired once, thedata packet containing the payload may be repeatedly transmitted,whereby a large amount of data may be transmitted.

FIG. 29 shows a transmission sequence used to continuously transmit alarge number of packets. FIG. 29 is a sequence diagram similar to FIG.27, wherein FIG. 29A shows a transmission sequence of the communicationstation STA1, FIG. 29B shows a transmission sequence of thecommunication station STA0 and FIGS. 29C, 29D show the states of thetransmission unit and the reception unit at the communication stationSTA0 (high level: active mode, low level: sleep mode).

At a time T0, the communication station STA1 starts transmitting thepacket. After that, let it be assumed that the high-order layer informsthe communication station STA0 that the communication station STA0 hasinformation to be transmitted (Tx request). Since the communicationstation STA0 is placed in the sleep mode immediately before the time T1,it start confirming that the media is clear during a time period fromthe time T1 to the MDI. Then, in order to detect the unique word(preamble) of the packet transmitted from the communication station STA1at the time T2, the communication station recognizes the existence ofthe packet transmitted from the communication station STA1. Thecommunication station STA0 starts the random backoff procedure from thetime T3 at which the transmission of this packet is ended. If thecommunication station does not detect the unique word until the time T4at which the timer is de-energized, then the communication stationtransmits the packet at the time T4.

While the values of the time, the space and the transmission rate havebeen described as examples thereof, the present invention is not limitedthereto, and it is needless to say that other values may be set to thosevalues without departing from the gist of the present invention.

Also, while the exclusive communication apparatus for performingtransmission and reception shown in FIG. 2 is constructed as thecommunication station in the above-mentioned embodiment, the presentinvention is not limited thereto and a board or a card for performingcommunication processing corresponding to the transmission unit and thereception unit in this embodiment may be attached to a personal computerapparatus for performing various data processing, for example, andsoftware executed by the side of the computer apparatus may be installedon the baseband unit for processing.

1-155. (canceled)
 156. A communication station that communicates withother communication stations as part of a wireless communication system,said communication station comprising: a receiver; a transmitter; and acontroller configured to control a receiving operation of said receiverfor a predetermined period of time after transmitting a signal from thetransmitter, wherein if another signal has not been transmitted from theother communication stations during said predetermined period of time,said controller stops the receiving operation until an occurrence of atleast one of a next signal receiving time and a next signal transmittingtime.
 157. The communication station according to claim 156, wherein thetransmitted signal includes beacon information.
 158. The communicationstation according to claim 156, wherein said controller is configured totransmit a beacon signal at approximately a constant time intervalrelative to transmission times of beacon signal transmissions from eachof said plurality of communication stations that are included in anetwork.
 159. The communication station according to claim 158, whereinthe controller is configured to continue the receiving operation overlonger than a predetermined beacon transmission time interval more thanonce during a time of a transmission frame.
 160. The communicationstation according to claim 156, wherein when the controller hasrecognized an impending occurrence of a beacon transmission scheduletime from another station by referencing an internal clock value, thecontroller orders the transmitter to transmit information for disablingtransmission from a peripheral station for a certain period of time.161. The communication station according to claim 156, wherein if thecontroller holds information therein addressed to another particularstation, the controller enables receiving processing by the receiver ata time when said another particular station transmits a beacon, and whenthe beacon transmission by said another particular station ends, thecontroller attempts transmission from the transmitter of the informationheld therein to said another particular station in accordance with aspecified procedure.
 162. The communication station according to claim161, wherein said information to be transmitted after an end of thebeacon transmission by said another particular station is informationhigher in priority than ordinary data.
 163. The communication stationaccording to claim 156, wherein, by detecting a signal transmitted fromanother station by operating the receiver for a predetermined period oftime before the transmission of a signal, access control is executed bythe controller so as to prevent a collision in packet communicationtiming with another station.
 164. The communication station according toclaim 163, wherein the controller is configured to make an attempt fortransmission immediately after a change from a sleep state to an activestate, when it is confirmed whether a media is clear for a period oftime equivalent to a specified maximum signal length.
 165. Thecommunication station according to claim 163, wherein, if the controllerholds information therein addressed to another particular station, thecontroller attempts to transmit the information held therein to saidanother particular station in accordance with a predetermined procedureimmediately before a beacon transmission time of said another particularstation.
 166. The communication station according to claim 163, whereinsaid information to be transmitted after an end of the beacontransmission by said another particular station is information higher inpriority than ordinary data.
 167. The communication station according toclaim 156, wherein, in transmitting information, the controller attemptstransmission to a station that has recognized that a transmissiondestination station is executing a receiving operation.
 168. Thecommunication station according to claim 156, wherein the controllerattempts to receive a beacon from another station recognized by thecontroller.
 169. The communication station according to claim 168,wherein said reception of a beacon from another station is executed inat least one of a continuous manner and an intermittent manner.
 170. Thecommunication station according to claim 168, wherein if the controllerholds information addressed to another particular station, thecontroller writes said information addressed to the another particularstation to a beacon to be transmitted therefrom, and a communicationstation that has received said beacon, upon recognition of saidinformation addressed thereto, transmits, to the receiver, a signalindicative that the information can be transmitted thereto.
 171. Thecommunication station according to claim 168, wherein, when controlleris located in an environment where a beacon can be received from anotherparticular station, if the controller is in a state where the controllerdoes not control communication with said another particular station,said controller does not attempt to receive a beacon that is transmittedfrom said another particular station.
 172. The communication stationaccording to claim 156, wherein, after transmitting a certain signal,the controller controls the receiver to perform a receiving operationfor a predetermined period of time and, if no signal addressed theretohas been received, the controller stops the receiving operation until atleast one of a next signal receiving schedule time and a next signaltransmitting schedule time.
 173. A communication station thatcommunicates with other communication stations as part of a wirelesscommunication system, said communication station comprising: a receiver;a transmitter; and means for controlling a receiving operation of saidreceiver for a predetermined period of time after transmitting a signalfrom the transmitter, and means for stopping the receiving operationuntil an occurrence of at lease one of a next signal receiving time anda next signal transmitting time if another signal has not beentransmitted from the other communication stations during saidpredetermined period of time.
 174. A method executed in a communicationstation that communicates with other communication stations as part of awireless communication system, said method comprising: receivingwireless signals with a receiver; transmitting wireless signals with atransmitter; and controlling with a controller a receiving operation ofsaid receiver for a predetermined period of time after transmitting asignal from the transmitter, wherein if another signal has not beentransmitted from the other communication stations during saidpredetermined period of time, said controller stops the receivingoperation until an occurrence of at least one of a next signal receivingtime and a next signal transmitting time.